feat: add Qobuz playlist follow/unfollow (#172)

Discovered undocumented Qobuz API endpoints from bundle analysis:
- POST /playlist/subscribe { playlist_id }
- POST /playlist/unsubscribe { playlist_id }

This is the native "follow playlist" feature that syncs across all
Qobuz clients, separate from QBZ's existing "Copy to Library" (local
copy) and "Favorite" (local flag) features.

Added:
- subscribe_playlist/unsubscribe_playlist in both qbz-qobuz and legacy client
- v2_qobuz_subscribe_playlist / v2_qobuz_unsubscribe_playlist commands
- Bookmark toggle button in PlaylistDetailView (next to Copy button)
- i18n keys in all 5 locales

cc

crates/qbz-qobuz/src/client.rs

@@ -1622,6 +1622,54 @@ impl QobuzClient {
         Ok(response)
     }
 
+    /// Subscribe to a Qobuz playlist (follow it in the user's library)
+    pub async fn subscribe_playlist(&self, playlist_id: u64) -> Result<()> {
+        let url = endpoints::build_url(paths::PLAYLIST_SUBSCRIBE);
+
+        let response = self
+            .http
+            .get(&url)
+            .headers(self.authenticated_headers().await?)
+            .query(&[("playlist_id", playlist_id.to_string())])
+            .send()
+            .await?;
+
+        let status = response.status();
+        if !status.is_success() {
+            let body = response.text().await.unwrap_or_default();
+            return Err(ApiError::ApiResponse(format!(
+                "playlist/subscribe failed ({}): {}",
+                status, body
+            )));
+        }
+
+        Ok(())
+    }
+
+    /// Unsubscribe from a Qobuz playlist
+    pub async fn unsubscribe_playlist(&self, playlist_id: u64) -> Result<()> {
+        let url = endpoints::build_url(paths::PLAYLIST_UNSUBSCRIBE);
+
+        let response = self
+            .http
+            .get(&url)
+            .headers(self.authenticated_headers().await?)
+            .query(&[("playlist_id", playlist_id.to_string())])
+            .send()
+            .await?;
+
+        let status = response.status();
+        if !status.is_success() {
+            let body = response.text().await.unwrap_or_default();
+            return Err(ApiError::ApiResponse(format!(
+                "playlist/unsubscribe failed ({}): {}",
+                status, body
+            )));
+        }
+
+        Ok(())
+    }
+
     /// Add item to favorites
     pub async fn add_favorite(&self, fav_type: &str, item_id: &str) -> Result<()> {
         let url = endpoints::build_url(paths::FAVORITE_CREATE);

crates/qbz-qobuz/src/endpoints.rs

@@ -39,6 +39,8 @@ pub mod paths {
     pub const PLAYLIST_DELETE_TRACKS: &str = "/playlist/deleteTracks";
     pub const PLAYLIST_UPDATE: &str = "/playlist/update";
     pub const PLAYLIST_GET_TAGS: &str = "/playlist/getTags";
+    pub const PLAYLIST_SUBSCRIBE: &str = "/playlist/subscribe";
+    pub const PLAYLIST_UNSUBSCRIBE: &str = "/playlist/unsubscribe";
 
     // Favorites
     pub const FAVORITE_GET_USER_FAVORITES: &str = "/favorite/getUserFavorites";