fix(frontend): a11y backdrop, ≥44px PIN button, test-ids on auth & upload

- account/+page.svelte: remove `aria-hidden="true"` from the
  leave-confirm and data-mode-warning bottom-sheet backdrops. The
  attribute cascaded into the dialog children, making the inner
  Abmelden/Aktivieren/Abbrechen buttons unreachable in the accessibility
  tree (and to Playwright's `getByRole`). Discovered while writing the
  E2E suite; the visual layout is unchanged.

- join/+page.svelte: bump the PIN-copy button from `py-1` (28px tall) to
  `min-h-11 min-w-11 py-2` so it clears the ≥44px touch-target floor on
  mobile. Touch-target audit revealed the gap.

- data-testid attributes on stable interactive elements (join name input,
  join submit, PIN modal + copy + continue, recovery PIN + submit + try-
  different-name, admin login password + submit + error, recover name +
  PIN + submit + error, upload header submit + sticky submit + caption
  textarea). Targeted at ~20 spots where semantic locators were ambiguous
  (e.g. two "Hochladen" buttons on /upload, German strings that may iterate).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/routes/account/+page.svelte

@@ -384,7 +384,7 @@
 
 <!-- Data-mode warning bottom sheet — shown once when the user picks Original. -->
 {#if dataModeWarningOpen}
-	<div class="fixed inset-0 z-50 flex items-end bg-black/40" onclick={() => (dataModeWarningOpen = false)} aria-hidden="true">
+	<div class="fixed inset-0 z-50 flex items-end bg-black/40" onclick={() => (dataModeWarningOpen = false)}>
 		<div
 			class="w-full rounded-t-2xl bg-white px-5 pb-10 pt-6 dark:bg-gray-900"
 			onclick={(e) => e.stopPropagation()}
@@ -418,7 +418,7 @@
 
 <!-- Leave-confirm bottom sheet -->
 {#if leaveConfirmOpen}
-	<div class="fixed inset-0 z-50 flex items-end bg-black/40" onclick={() => (leaveConfirmOpen = false)} aria-hidden="true">
+	<div class="fixed inset-0 z-50 flex items-end bg-black/40" onclick={() => (leaveConfirmOpen = false)}>
 		<div
 			class="w-full rounded-t-2xl bg-white px-5 pb-10 pt-6 dark:bg-gray-900"
 			onclick={(e) => e.stopPropagation()}

frontend/src/routes/admin/login/+page.svelte

@@ -45,16 +45,18 @@
 				bind:value={password}
 				placeholder="Passwort"
 				autocomplete="current-password"
+				data-testid="admin-password-input"
 				class="mb-3 w-full rounded-lg border border-gray-300 bg-white px-4 py-3 text-lg text-gray-900 placeholder-gray-400 focus:border-blue-500 focus:outline-none focus:ring-2 focus:ring-blue-200 dark:border-gray-700 dark:bg-gray-900 dark:text-gray-100 dark:placeholder-gray-500"
 			/>
 
 			{#if error}
-				<p class="mb-3 text-sm text-red-600 dark:text-red-400">{error}</p>
+				<p class="mb-3 text-sm text-red-600 dark:text-red-400" data-testid="admin-login-error">{error}</p>
 			{/if}
 
 			<button
 				type="submit"
 				disabled={loading || !password}
+				data-testid="admin-login-submit"
 				class="w-full rounded-lg bg-blue-600 px-4 py-3 text-lg font-medium text-white transition hover:bg-blue-700 disabled:opacity-50 dark:bg-blue-500 dark:hover:bg-blue-400"
 			>
 				{loading ? 'Wird angemeldet…' : 'Anmelden'}

frontend/src/routes/join/+page.svelte

@@ -111,16 +111,18 @@
 					maxlength={4}
 					inputmode="numeric"
 					pattern="[0-9]*"
+					data-testid="recovery-pin-input"
 					class="mb-3 w-full rounded-lg border border-gray-300 bg-white px-4 py-3 text-center text-2xl font-mono tracking-widest text-gray-900 focus:border-blue-500 focus:outline-none focus:ring-2 focus:ring-blue-200 dark:border-gray-700 dark:bg-gray-900 dark:text-gray-100"
 				/>
 
 				{#if recoveryError}
-					<p class="mb-3 text-sm text-red-600 dark:text-red-400">{recoveryError}</p>
+					<p class="mb-3 text-sm text-red-600 dark:text-red-400" data-testid="recovery-error">{recoveryError}</p>
 				{/if}
 
 				<button
 					type="submit"
 					disabled={recoveryLoading || recoveryPin.length < 4}
+					data-testid="recovery-submit"
 					class="mb-3 w-full rounded-lg bg-blue-600 px-4 py-3 font-medium text-white transition hover:bg-blue-700 disabled:opacity-50 dark:bg-blue-500 dark:hover:bg-blue-400"
 				>
 					{recoveryLoading ? 'Wird angemeldet...' : 'Anmelden'}
@@ -129,6 +131,7 @@
 
 			<button
 				onclick={tryDifferentName}
+				data-testid="try-different-name"
 				class="w-full rounded-lg border border-gray-300 px-4 py-3 font-medium text-gray-700 transition hover:bg-gray-50 dark:border-gray-700 dark:text-gray-300 dark:hover:bg-gray-800"
 			>
 				Anderen Namen wählen
@@ -145,16 +148,18 @@
 					bind:value={displayName}
 					placeholder="Dein Name"
 					maxlength={50}
+					data-testid="join-name-input"
 					class="mb-3 w-full rounded-lg border border-gray-300 bg-white px-4 py-3 text-lg text-gray-900 placeholder-gray-400 focus:border-blue-500 focus:outline-none focus:ring-2 focus:ring-blue-200 dark:border-gray-700 dark:bg-gray-900 dark:text-gray-100 dark:placeholder-gray-500"
 				/>
 
 				{#if error}
-					<p class="mb-3 text-sm text-red-600 dark:text-red-400">{error}</p>
+					<p class="mb-3 text-sm text-red-600 dark:text-red-400" data-testid="join-error">{error}</p>
 				{/if}
 
 				<button
 					type="submit"
 					disabled={loading || !displayName.trim()}
+					data-testid="join-submit"
 					class="w-full rounded-lg bg-blue-600 px-4 py-3 text-lg font-medium text-white transition hover:bg-blue-700 disabled:opacity-50 dark:bg-blue-500 dark:hover:bg-blue-400"
 				>
 					{loading ? 'Wird geladen...' : 'Beitreten'}
@@ -162,7 +167,7 @@
 			</form>
 
 			<p class="mt-4 text-center text-sm">
-				<a href="/recover" class="text-blue-600 hover:underline dark:text-blue-400">Ich habe bereits einen Account</a>
+				<a href="/recover" data-testid="link-to-recover" class="text-blue-600 hover:underline dark:text-blue-400">Ich habe bereits einen Account</a>
 			</p>
 		{/if}
 
@@ -170,7 +175,7 @@
 </div>
 
 {#if showPinModal}
-	<div class="fixed inset-0 z-50 flex items-center justify-center bg-black/50 px-4">
+	<div class="fixed inset-0 z-50 flex items-center justify-center bg-black/50 px-4" data-testid="pin-modal">
 		<div class="w-full max-w-sm rounded-xl bg-white p-6 shadow-lg dark:bg-gray-900">
 			<h2 class="mb-2 text-xl font-bold text-gray-900 dark:text-gray-100">Dein Wiederherstellungs-PIN</h2>
 			<p class="mb-4 text-sm text-gray-600 dark:text-gray-400">
@@ -178,10 +183,11 @@
 			</p>
 
 			<div class="mb-4 flex items-center justify-center gap-3 rounded-lg bg-gray-100 p-4 dark:bg-gray-800">
-				<span class="text-4xl font-mono font-bold tracking-widest text-gray-900 dark:text-gray-100">{pin}</span>
+				<span class="text-4xl font-mono font-bold tracking-widest text-gray-900 dark:text-gray-100" data-testid="pin-display">{pin}</span>
 				<button
 					onclick={copyPin}
-					class="rounded-md bg-gray-200 px-3 py-1 text-sm font-medium text-gray-700 hover:bg-gray-300 dark:bg-gray-700 dark:text-gray-200 dark:hover:bg-gray-600"
+					data-testid="pin-copy"
+					class="min-h-11 min-w-11 rounded-md bg-gray-200 px-3 py-2 text-sm font-medium text-gray-700 hover:bg-gray-300 dark:bg-gray-700 dark:text-gray-200 dark:hover:bg-gray-600"
 				>
 					{copied ? 'Kopiert!' : 'Kopieren'}
 				</button>
@@ -189,6 +195,7 @@
 
 			<button
 				onclick={goToFeed}
+				data-testid="continue-to-feed"
 				class="w-full rounded-lg bg-blue-600 px-4 py-3 font-medium text-white transition hover:bg-blue-700 dark:bg-blue-500 dark:hover:bg-blue-400"
 			>
 				Weiter zur Galerie

frontend/src/routes/recover/+page.svelte

@@ -50,6 +50,7 @@
 				bind:value={displayName}
 				placeholder="Dein Name"
 				maxlength={50}
+				data-testid="recover-name-input"
 				class="mb-3 w-full rounded-lg border border-gray-300 bg-white px-4 py-3 text-lg text-gray-900 placeholder-gray-400 focus:border-blue-500 focus:outline-none focus:ring-2 focus:ring-blue-200 dark:border-gray-700 dark:bg-gray-900 dark:text-gray-100 dark:placeholder-gray-500"
 			/>
 			<input
@@ -59,16 +60,18 @@
 				maxlength={4}
 				inputmode="numeric"
 				pattern="[0-9]*"
+				data-testid="recover-pin-input"
 				class="mb-3 w-full rounded-lg border border-gray-300 bg-white px-4 py-3 text-center text-2xl font-mono tracking-widest text-gray-900 placeholder-gray-400 focus:border-blue-500 focus:outline-none focus:ring-2 focus:ring-blue-200 dark:border-gray-700 dark:bg-gray-900 dark:text-gray-100 dark:placeholder-gray-500"
 			/>
 
 			{#if error}
-				<p class="mb-3 text-sm text-red-600 dark:text-red-400">{error}</p>
+				<p class="mb-3 text-sm text-red-600 dark:text-red-400" data-testid="recover-error">{error}</p>
 			{/if}
 
 			<button
 				type="submit"
 				disabled={loading || !displayName.trim() || pin.length < 4}
+				data-testid="recover-submit"
 				class="w-full rounded-lg bg-blue-600 px-4 py-3 text-lg font-medium text-white transition hover:bg-blue-700 disabled:opacity-50 dark:bg-blue-500 dark:hover:bg-blue-400"
 			>
 				{loading ? 'Wird geladen...' : 'Wiederherstellen'}

frontend/src/routes/upload/+page.svelte

@@ -119,6 +119,7 @@
 		<button
 			onclick={handleSubmit}
 			disabled={stagedFiles.length === 0 || submitting}
+			data-testid="upload-submit-header"
 			class="rounded-lg bg-blue-600 px-4 py-1.5 text-sm font-semibold text-white transition
 				hover:bg-blue-700 disabled:opacity-40 dark:bg-blue-500 dark:hover:bg-blue-400"
 		>
@@ -179,6 +180,7 @@
 				bind:this={captionEl}
 				bind:value={caption}
 				maxlength={MAX_CAPTION_LENGTH}
+				data-testid="upload-caption"
 				placeholder="Beschreibung hinzufügen… (#hashtags möglich)"
 				rows="4"
 				class="w-full resize-none rounded-xl border border-gray-200 bg-gray-50 px-4 py-3 text-sm text-gray-900
@@ -230,6 +232,7 @@
 		<button
 			onclick={handleSubmit}
 			disabled={stagedFiles.length === 0 || submitting}
+			data-testid="upload-submit"
 			class="flex w-full items-center justify-center gap-2 rounded-xl bg-blue-600 py-3.5 text-sm font-semibold
 				text-white transition hover:bg-blue-700 active:scale-[0.98] disabled:opacity-40 dark:bg-blue-500 dark:hover:bg-blue-400"
 		>