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/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"
 		>