Credential enable/disable + descriptions, artist casing settings page, playlist compare view

Credentials page now shows a fact-checked one-line description per
service and marks Spotify/Soulseek/Navidrome as Required (no toggle --
they're load-bearing for every playlist sync). The four optional
integrations (Bandcamp, AzuraCast, Qobuz, Telegram) get a real on/off
switch: Bandcamp pauses its scheduled job (same mechanism as the Jobs
page), the other three just remove their rendered credential file, which
the scripts that read them already treat as "not configured, skip" --
no script changes needed. Disabled optional scopes also drop off the
dashboard's credential status row.

New Settings > Artist Casing page to view/add/remove entries in
artist-canonical.list without shelling in -- adding a name that already
matches case-insensitively replaces its casing in place instead of
duplicating.

Playlist detail page's status section is now two side-by-side columns:
the original Spotify tracklist on the left, and a tabbed Downloaded /
Waiting-to-download view on the right (client-side tabs via Alpine),
both with an internal scroll area so long playlists don't pull the
columns out of sync. Uses the existing status_service reconciliation
(sldl index + beets + quarantine) -- no backend changes needed there.

app/main.py

@@ -5,7 +5,7 @@ from fastapi.staticfiles import StaticFiles
 from starlette.middleware.sessions import SessionMiddleware
 
 from app.db import enable_beets_db_wal, init_db
-from app.routers import auth, credentials, dashboard, dedup, genres, import_, jobs, library, playlists
+from app.routers import artist_casing, auth, credentials, dashboard, dedup, genres, import_, jobs, library, playlists
 from app.services import scheduler_service
 from app.settings import settings
 
@@ -40,6 +40,7 @@ def create_app() -> FastAPI:
     app.include_router(playlists.router)
     app.include_router(credentials.router)
     app.include_router(jobs.router)
+    app.include_router(artist_casing.router)
 
     return app