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/src/main.rs

@@ -25,12 +25,13 @@ const SEARCH_LIMIT: u32 = 25;
 // `ytplayer.exe` is fully self-contained — no sibling file and nothing on PATH
 // required. On first run we materialize it into the app cache dir and run from there.
 // (Windows only; other platforms fall back to a resource/dev/PATH copy.)
-#[cfg(windows)]
+// Disable with --no-default-features for faster debug builds.
+#[cfg(all(windows, feature = "embed-ytdlp"))]
 static YTDLP_EMBEDDED: &[u8] = include_bytes!("../../bin/yt-dlp.exe");
 
 /// Write the embedded yt-dlp to the cache dir if it isn't already there (size-checked),
 /// and return its path.
-#[cfg(windows)]
+#[cfg(all(windows, feature = "embed-ytdlp"))]
 fn embedded_ytdlp(app: &tauri::AppHandle) -> Option<PathBuf> {
     let dir = app.path().app_cache_dir().ok()?;
     std::fs::create_dir_all(&dir).ok()?;
@@ -58,7 +59,7 @@ fn ytdlp_path(app: &tauri::AppHandle) -> PathBuf {
     if dev.exists() {
         return dev;
     }
-    #[cfg(windows)]
+    #[cfg(all(windows, feature = "embed-ytdlp"))]
     {
         if let Some(p) = embedded_ytdlp(app) {
             return p;