fix(12-02): wait for scan to stop before library removal, surface errors in UI

RemoveLibrary now polls until the cancelled scan goroutine finishes
before proceeding with the removal transaction. Also show removal
errors as toast messages instead of only logging to console, and
explicitly reload library list after successful removal.

frontend/src/components/config-page/config-page.ts

@@ -1396,9 +1396,13 @@ export class ConfigPage extends LitElement {
             this.showToast(
                 `Removed '${libName}' (${summary?.tracksDeleted ?? 0} tracks deleted)`,
             );
+            void this.loadLibraries();
         } catch (err) {
             this.isRemoving = false;
+            this.removingLibraryId = null;
+            this.removalImpact = null;
             console.error('Failed to remove library:', err);
+            this.showToast(`Failed to remove '${libName}': ${String(err)}`);
         }
     };