feat: playlist management, gapless playback, ReplayGain, Qobuz theme

Playlist management:
- Add/remove tracks from playlists via right-click context menu
- Create new playlists (right-click Playlists sidebar header)
- Delete playlists with confirmation dialog (right-click playlist item)
- Playlist view removes track immediately on delete (optimistic)
- Deleting currently-open playlist clears the track view

Gapless playback:
- Single long-running audio thread owns AudioOutput; CPAL stream stays
  open between tracks eliminating device teardown/startup gap
- Decode runs inline on the audio thread; command channel polled via
  try_recv() so Pause/Resume/Seek/Stop/Play all work without spawning
- New Play command arriving mid-decode is handled immediately,
  reusing the same audio output for zero-gap transition
- Position timer reduced from 500 ms to 50 ms for faster track-end detection
- URL/metadata prefetch: when gapless is enabled Qt pre-fetches the next
  track while the current one is still playing

ReplayGain:
- Toggled in Settings → Playback
- replaygain_track_gain (dB) from track audio_info converted to linear
  gain factor and applied per-sample alongside volume

Qobuz dark theme:
- Background #191919, base #141414, accent #FFB232 (yellow-orange)
- Selection highlight, slider fill, scrollbar hover all use #FFB232
- Links use Qobuz blue #46B3EE
- Hi-res H badges updated to #FFB232 (from #FFD700)
- Now-playing row uses #FFB232 (was Spotify green)
- QSS stylesheet for scrollbars, menus, inputs, buttons, groups

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

rust/src/api/client.rs

@@ -89,6 +89,15 @@ impl QobuzClient {
         Ok(body)
     }
 
+    fn post_request(&self, method: &str) -> reqwest::RequestBuilder {
+        let mut builder = self.http.post(self.url(method));
+        builder = builder.query(&[("app_id", self.app_id.as_str())]);
+        if let Some(token) = &self.auth_token {
+            builder = builder.header("Authorization", format!("Bearer {}", token));
+        }
+        builder
+    }
+
     fn get_request(&self, method: &str) -> reqwest::RequestBuilder {
         let mut builder = self.http.get(self.url(method));
         builder = builder.query(&[("app_id", self.app_id.as_str())]);
@@ -329,6 +338,55 @@ impl QobuzClient {
         Ok(serde_json::from_value(body["artists"].clone())?)
     }
 
+    // --- Playlist management ---
+
+    pub async fn create_playlist(&self, name: &str) -> Result<PlaylistDto> {
+        let resp = self
+            .post_request("playlist/create")
+            .form(&[("name", name), ("is_public", "false"), ("is_collaborative", "false")])
+            .send()
+            .await?;
+        let body = Self::check_response(resp).await?;
+        Ok(serde_json::from_value(body)?)
+    }
+
+    pub async fn add_track_to_playlist(&self, playlist_id: i64, track_id: i64) -> Result<()> {
+        let resp = self
+            .post_request("playlist/addTracks")
+            .form(&[
+                ("playlist_id", playlist_id.to_string()),
+                ("track_ids",   track_id.to_string()),
+                ("no_duplicate", "true".to_string()),
+            ])
+            .send()
+            .await?;
+        Self::check_response(resp).await?;
+        Ok(())
+    }
+
+    pub async fn delete_playlist(&self, playlist_id: i64) -> Result<()> {
+        let resp = self
+            .get_request("playlist/delete")
+            .query(&[("playlist_id", &playlist_id.to_string())])
+            .send()
+            .await?;
+        Self::check_response(resp).await?;
+        Ok(())
+    }
+
+    pub async fn delete_track_from_playlist(&self, playlist_id: i64, playlist_track_id: i64) -> Result<()> {
+        let resp = self
+            .post_request("playlist/deleteTracks")
+            .form(&[
+                ("playlist_id",        playlist_id.to_string()),
+                ("playlist_track_ids", playlist_track_id.to_string()),
+            ])
+            .send()
+            .await?;
+        Self::check_response(resp).await?;
+        Ok(())
+    }
+
     pub async fn add_fav_track(&self, track_id: i64) -> Result<()> {
         let resp = self
             .get_request("favorite/create")