Add web themes, network binding, UI auth, library maintenance; optimize + fix rescan

Features:
- Port all 7 desktop themes (Dracula, Trans, Emo variants, etc.) to the web UI
- Configurable bind interface (localhost/Tailscale/LAN/all) with detected IPs,
  settable from both the web UI and desktop GUI
- Optional password that gates the whole UI and all API access via session
  cookies (Argon2), with a login page, logout, and middleware; settable from
  both UIs
- Library maintenance: scan for duplicate video IDs and missing assets
  (thumbnail/info.json/description), remove duplicates (scan-and-confirm, with
  out-of-library path guard), and re-fetch missing sidecars via yt-dlp.
  Surfaced in both the web UI and a desktop GUI window.

Optimizations / cleanup:
- Cache has_chapters at scan time (library.rs) instead of re-reading and
  parsing every info.json on each /api/library request
- Use the cached channel size in get_library
- Pool a single SQLite connection in WebState instead of opening one per request
- Remove a 1.2 GB stray nested src/yt-offline duplicate tree

Fixes:
- Rescan no longer clears all thumbnail textures (caused thumbnails to unload);
  prune only removed entries and bump library_generation so the card grid
  refreshes correctly

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Cargo.toml

@@ -17,6 +17,8 @@ tokio = { version = "1", features = ["full"] }
 tokio-util = { version = "0.7", features = ["io"] }
 tokio-stream = "0.1"
 tower-http = { version = "0.5", features = ["cors", "fs"] }
+argon2 = "0.5"
+rand = "0.8"
 
 [profile.release]
 opt-level = 2