feat: add multi-provider support (Komga + Stripstream Librarian)

- Introduce provider abstraction layer (IMediaProvider, KomgaProvider, StripstreamProvider)
- Add Stripstream Librarian as second media provider with full feature parity
- Migrate all pages and components from direct Komga services to provider factory
- Remove dead service code (BaseApiService, HomeService, LibraryService, SearchService, TestService)
- Fix library/series page-based pagination for both providers (Komga 0-indexed, Stripstream 1-indexed)
- Fix unread filter and search on library page for both providers
- Fix read progress display for Stripstream (reading_status mapping)
- Fix series read status (books_read_count) for Stripstream
- Add global search with series results for Stripstream (series_hits from Meilisearch)
- Fix thumbnail proxy to return 404 gracefully instead of JSON on upstream error
- Replace duration-based cache debug detection with x-nextjs-cache header

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

src/i18n/messages/en/common.json

@@ -377,6 +377,9 @@
     "KOMGA_CONNECTION_ERROR": "Error connecting to Komga server",
     "KOMGA_HTTP_ERROR": "HTTP error while communicating with Komga",
     "KOMGA_SERVER_UNREACHABLE": "Komga server unreachable",
+    "STRIPSTREAM_MISSING_CONFIG": "Stripstream Librarian configuration missing",
+    "STRIPSTREAM_CONNECTION_ERROR": "Error connecting to Stripstream Librarian",
+    "STRIPSTREAM_HTTP_ERROR": "HTTP error while communicating with Stripstream Librarian",
 
     "CONFIG_SAVE_ERROR": "Error saving configuration",
     "CONFIG_FETCH_ERROR": "Error fetching configuration",