Add cache cap, playlist export/import, yt-dlp build flag

Cache cap: dropdown setting (no limit / 1/2/5/10 GB) stored in
settings.cacheCap (actual eviction handled by native side).
Export: downloads playlists as JSON via Blob. Import: file reader
merges new playlists by id. yt-dlp: feature gate 'embed-ytdlp'
(default on), skip with --no-default-features for faster debug builds.

Files:
- frontend/app.js: exportPlaylists/importPlaylists, cache cap select
- src-tauri/Cargo.toml: [features] embed-ytdlp
- src-tauri/src/main.rs: #[cfg(feature = embed-ytdlp)] guards

src-tauri/Cargo.toml

@@ -14,6 +14,10 @@ tauri = { version = "2", features = ["protocol-asset"] }
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 
+[features]
+default = ["embed-ytdlp"]
+embed-ytdlp = []
+
 [profile.release]
 opt-level = "s"
 lto = true