chore(docs): implement documentation

docs/user/albums.md

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+## Albums
+
+Open an album from search or artist page.
+
+- Download
+  - Download full album
+  - Download individual tracks from the tracklist
+- Tracklist
+  - Shows order, artists, and duration
+  - Respects explicit filter (hidden if enabled)
+- Large albums
+  - Tracks load progressively as you scroll
+
+Backend endpoints used:
+- GET `/api/album/info?id=...&limit=50&offset=...` (metadata + paged tracks)
+- GET `/api/album/download/{album_id}` (queue download)
+- GET `/api/progress/stream` (live queue updates)