feat(slint): searchType filter dropdown for per-type tabs

The Albums / Tracks / Artists tabs get a QbzSelect filter (No filter /
Main artist / Performer / Composer / Label / Release name). Changing it
re-queries that category from offset 0 with the search_type and replaces
the list; load-more carries the active filter forward. The filter
resets on every new search. search_type_for_filter is unit-tested.

cc

crates/qbz-slint/ui/search/SearchResultsView.slint

@@ -15,6 +15,7 @@ import { SearchTrackRow } from "SearchTrackRow.slint";
 import { QbzIcon } from "../primitives/QbzIcon.slint";
 import { ContextMenu } from "../primitives/ContextMenu.slint";
 import { ContextMenuItem } from "../primitives/ContextMenuItem.slint";
+import { QbzSelect } from "../primitives/QbzSelect.slint";
 
 // One tab in the results tab strip.
 component SearchTab inherits Rectangle {
@@ -375,6 +376,26 @@ export component SearchResultsView inherits Rectangle {
             }
         }
 
+        // searchType filter — shown on the Albums / Tracks / Artists tabs.
+        if SearchState.tab >= 1 && SearchState.tab <= 3: HorizontalLayout {
+            alignment: start;
+            QbzSelect {
+                menu-width: 180px;
+                options: [
+                    @tr("No filter"),
+                    @tr("Main artist"),
+                    @tr("Performer"),
+                    @tr("Composer"),
+                    @tr("Label"),
+                    @tr("Release name"),
+                ];
+                current-index: SearchState.filter-index;
+                selected(i) => {
+                    SearchActions.filter-changed(i);
+                }
+            }
+        }
+
         // Scrollable results region.
         Flickable {
             vertical-stretch: 1;

crates/qbz-slint/ui/state.slint

@@ -372,8 +372,9 @@ export global SearchState {
     in property <AlbumCardItem> most-popular-album;
     in property <SlimItem> most-popular-artist;
     in property <SearchTrackItem> most-popular-track;
-    // Active searchType filter: "" | "MainArtist" | "Performer" | ...
-    in property <string> filter-type;
+    // Active searchType filter, as a dropdown index (0 = no filter).
+    // The controller owns the index -> API-value mapping.
+    in property <int> filter-index: 0;
 }
 
 // Search actions wired to the Rust search controller.
@@ -384,8 +385,8 @@ export global SearchActions {
     callback live(string);
     // User picked a results tab (0..4).
     callback tab-changed(int);
-    // searchType filter changed.
-    callback filter-changed(string);
+    // searchType filter changed — the new dropdown index.
+    callback filter-changed(int);
     // Request the next page for the given tab.
     callback load-more(int);
 }