Implement public ShareViewerScreen with Android deep-link intent-filter (9b)

- New ShareViewerScreen (/share/:token): unauthenticated share-viewer that
  fetches share metadata via publicApiClientProvider, renders filename, type,
  duration, thumbnail, and a Play button routing to the video/audio player.
- New publicApiClientProvider: bare Dio client (no auth interceptors) for the
  public share endpoint; shares kPlayerBaseUrl with the authenticated client.
- AndroidManifest: http + https deep-link intent-filters for /share/.* so
  Android routes share URLs directly into the app (App Links / autoVerify).
- router.dart: /share/:token bypasses the authentication redirect; guard uses
  AppRoutes.shareViewerPrefix constant instead of a raw '/share/' string (DIP).
- app_routes.dart: shareViewer route constant, shareViewerPrefix, shareViewerPath helper.
- error_mappers.dart: shareViewerErrorMessage — 404 invalid/revoked, 410 expired.
- player_api_client.dart: baseUrl getter encapsulates rawDio.options.baseUrl so
  screens never access transport internals directly (ISP, DIP).
- Review fixes: OCP icon map in _FallbackThumbnail, LSP explicit baseUrl
  overrides in test fakes, DIP shareViewerPrefix constant.

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

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