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()}