feat(dashboard): bulk-create collections from an addon's catalogs

[bigverm23]

What

Adds a bulk-import workflow for addon catalogs. Today, turning a
Stremio addon's catalogs into collections means creating them one at
a time through the manual "New Collection" form and hand-picking the
catalog filter. This adds a dedicated screen for it.

New screen: Addons → Catalogs

• Click "Catalogs" on any addon that exposes the catalog resource
  to see all its catalogs in a grid, each tagged with its media kind
  (Movies/Shows/Mixed/Music/Collections).
• Search box to filter the grid by name.
• Select multiple catalogs and either:
  • Separate collections — one collection per catalog, each
    filtered to just that catalog's items.
  • Merge into one — combine all selected catalogs into a
    single named collection.
• Optional overrides before creating: explicit Media Kind
  (instead of relying on auto-detection) and Tags, applied to
  whatever gets created.
• Catalogs that already have a collection (tracked via the new
  collection_source field) are greyed out with an "Already added"
  badge, so re-visiting the screen won't offer duplicates.

Backend changes

• AddonCatalogDto gains collection_media_kind, populated in
  get_addon_catalogs, so the dashboard knows each catalog's kind
  without guessing.
• PatchItemPayload gains collection_source
  ("{addon_uuid}:{local_catalog_id}"), persisted by patch_item,
  so catalog-derived collections carry a pointer back to their
  source catalog. Already surfaced via the existing
  remux.collectionSource field on item responses.

Testing

• cargo check across remux-sdks, remux-server, remux-dashboard
• Ran locally via cargo make dev, walked through: creating separate
  collections, creating a merged collection, re-visiting the screen
  to confirm dedup/greying, media-kind override, and tags.

Notes / follow-ups I did not attempt here

• CollectionMediaKind::Music (server) vs MediaKind::Track (SDK)
  don't line up, so a music catalog's kind won't round-trip perfectly
  today — same pre-existing gap as in api/models.rs. Worth a
  separate fix.
• No bulk-delete / bulk-manage for existing catalog-sourced
  collections yet — this PR is additive/creation-only.

The initial implementation was drafted with Claude and then reviewed
and tested locally by me.