Remove YouTube integration from web-player; unify Home track rows

Drops every YouTube-backed route, proxy and type (external
albums/tracks, thumbnail proxy, youtube.ts) and simplifies
player/track/mix types to the local-only catalog now that the
backend contract dropped Source/ExternalId. Genre tiles on Explore
and "Populares esta semana" on Home stay as deliberate client-only
placeholders per product decision, just without the YouTube backing.

Also finishes the design-system unification pass: Home's three
hand-rolled track rows (Continuar escuchando, Playlist destacada,
Populares esta semana) now reuse the shared TrackTitleCell component
instead of duplicating cover+title+artist markup, and the orphaned
Rail.svelte (no importers) is removed.

web-player/.env.example

@@ -26,17 +26,6 @@ API_BASE_URL=http://localhost:5111
 # Dev already allows a plain-HTTP issuer. Set this only to allow it in a production build (never with a public issuer).
 # AUTH_ALLOW_INSECURE_HTTP=true
 
-# Optional tuning for the YouTube artwork proxy (/api/youtube/thumbnail),
-# which caches the covers in memory so the browser never hits Google directly.
-# THUMBNAIL_CACHE_MAX_AGE_SECONDS=2592000
-# THUMBNAIL_FAILURE_CACHE_SECONDS=300
-# THUMBNAIL_CACHE_BYTES=134217728
-# THUMBNAIL_MAX_CONCURRENT_FETCHES=4
-# THUMBNAIL_FETCH_TIMEOUT_MS=8000
-# THUMBNAIL_COOLDOWN_SECONDS=30
-# THUMBNAIL_MAX_COOLDOWN_SECONDS=300
-# THUMBNAIL_MAX_SIZE=720
-
 # Where to send presigned S3 PUTs when the public S3 host is not reachable from
 # the server (in Docker: http://seaweedfs:8333). The signed Host header is kept
 # either way. Defaults to the host in the presigned URL.