feat: most-popular search as default Top Results tab

Add /most-popular/get endpoint support (discovered via Burp):
- Returns mixed artists/albums/tracks ordered by popularity for a query
- New 'Top Results' tab shown first/default in the search panel
- Badge column: A (artist, blue), H (hi-res album, gold), A (album, gray), T (track, green)
- Full context menu support on all item types in the new tab
- Both most-popular and regular search fire on submit in parallel

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

rust/src/lib.rs

@@ -70,6 +70,7 @@ pub const EV_TRACK_URL_ERR: c_int = 18;
 pub const EV_GENERIC_ERR: c_int = 19;
 pub const EV_ARTIST_RELEASES_OK: c_int = 24;
 pub const EV_DEEP_SHUFFLE_OK: c_int = 25;
+pub const EV_MOST_POPULAR_OK: c_int = 26;
 
 // ---------- Callback ----------
 
@@ -222,6 +223,28 @@ pub unsafe extern "C" fn qobuz_backend_search(
     });
 }
 
+#[no_mangle]
+pub unsafe extern "C" fn qobuz_backend_most_popular_search(
+    ptr: *mut Backend,
+    query: *const c_char,
+    limit: u32,
+) {
+    let inner = &(*ptr).0;
+    let query = CStr::from_ptr(query).to_string_lossy().into_owned();
+    let client = inner.client.clone();
+    let cb = inner.cb;
+    let ud = inner.ud;
+
+    spawn(inner, async move {
+        let result = client.lock().await.most_popular_search(&query, limit).await;
+        let (ev, json) = match result {
+            Ok(r) => (EV_MOST_POPULAR_OK, serde_json::to_string(&r).unwrap_or_default()),
+            Err(e) => (EV_GENERIC_ERR, err_json(&e.to_string())),
+        };
+        call_cb(cb, ud, ev, &json);
+    });
+}
+
 // ---------- Album ----------
 
 #[no_mangle]