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/DuplicateFinder.tsx

@@ -1,6 +1,7 @@
 import { useState } from "react";
 import { convertFileSrc } from "@tauri-apps/api/core";
 import { DuplicateGroup, useGalleryStore } from "../store";
+import { FolderScopeDropdown } from "./FolderScopeDropdown";
 
 function formatBytes(bytes: number): string {
   if (bytes >= 1_073_741_824) return `${(bytes / 1_073_741_824).toFixed(1)} GB`;
@@ -188,6 +189,7 @@ export function DuplicateFinder() {
             )}
           </div>
           <div className="flex items-center gap-2">
+            <FolderScopeDropdown />
             {/* Batch select — only shown when there are groups and nothing is selected yet */}
             {hasResults && selectedCount === 0 && !deleting && (
               <button