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/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'}