feat(views): in-view folder scope switching

Timeline, Explore, and Duplicates get a folder-scope dropdown in their
headers (Timeline via the shared Toolbar), so changing scope no longer
means leaving the feature and bouncing through the sidebar. The new
setViewFolderScope store action updates selectedFolderId while
preserving activeView: Timeline/Gallery reload images, Explore reloads
via its existing selectedFolderId effect, and Duplicates loads the
cached results for the new scope (fresh scans remain an explicit
Rescan). Sidebar behavior is unchanged — clicking a library still
opens its gallery.

src/components/Toolbar.tsx

@@ -1,6 +1,7 @@
 import { useEffect, useRef, useState } from "react";
 import { invoke } from "@tauri-apps/api/core";
 import { tileSizeForZoom, useGalleryStore, SortOrder, MediaFilter, SearchCommand, parseSearchValue, searchModeLabel, ExploreTagEntry } from "../store";
+import { FolderScopeDropdown } from "./FolderScopeDropdown";
 
 const BASE_SORT_OPTIONS: { value: SortOrder; label: string }[] = [
   { value: "date_desc", label: "Newest first" },
@@ -162,6 +163,7 @@ export function Toolbar() {
   const mediaJobProgress = useGalleryStore((state) => state.mediaJobProgress);
   const zoomPreset = useGalleryStore((state) => state.zoomPreset);
   const setZoomPreset = useGalleryStore((state) => state.setZoomPreset);
+  const activeView = useGalleryStore((state) => state.activeView);
 
   const hasAnyFailedEmbeddings = Object.values(mediaJobProgress).some((p) => p.embedding_failed > 0);
 
@@ -269,6 +271,8 @@ export function Toolbar() {
           )}
         </div>
 
+        {activeView === "timeline" ? <FolderScopeDropdown /> : null}
+
         <div className="flex-1" />
 
         {/* Search */}