[sang765]

Summary

Adds a "Migrate sources" action to the Library (favourites) menu that migrates all favourites from one source to another in bulk:

1. Pick the source from which to migrate (only sources present in favourites are listed)
2. Pick the target source (all known sources except LOCAL/TEST/UNKNOWN and the source itself)
3. Confirm dialog shows the manga count, then a foreground service migrates everything in the background with a progress notification and a final summary (migrated / no match / failed)

How it works

Reuses the existing per-manga migration pipeline:

• MigrateToSourceUseCase (new) — looks up the manga in the DB, finds the best match on the target source via SearchV2Helper (title search, relevance-sorted, alt-title fallback), then delegates to MigrateUseCase (favourites + history + tracks + scrobblers are moved)
• MigrateSourceService (new) — CoroutineIntentService modeled after AutoFixService; processes the batch sequentially (gentle on source servers), reports progress via notification
• UI entry point: opt_favourites_container.xml + dialog flow in FavouritesContainerFragment

Known limitations

• Page bookmarks and downloaded chapters are not migrated — same as the existing per-manga MigrateUseCase
• Manga that cannot be found on the target source are left untouched (counted as "no match" in the summary)
• New strings added for English and Vietnamese; other languages left to Weblate

Test plan

• spotlessApply passes (ran locally)
• Build + run on device: migrate a small source, verify favourites/history/reading progress moved and unmapped manga untouched
• Verify cancellation via the notification action stops the batch