Implement AdminPermissionsScreen, RescanScreen, and TrashScreen (eb)

Add three admin-only screens with routes and navigation tiles:
- AdminPermissionsScreen: permission matrix (users × sets) with optimistic
  grant/revoke checkboxes; admin rows shown as disabled (implicit access).
- AdminRescanScreen: trigger library rescan, poll getScanProgress every 2s
  while running, cancel timer in dispose, show live file/set counters.
- AdminTrashScreen: list soft-deleted media with restore + hard-delete
  (confirmation dialog); optimistic UI with revert-on-error for both actions.
All three use generation counters for stale-async cancellation and guard
async continuations on mounted. Added error mappers and route constants.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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