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/TagCloud.tsx

@@ -2,6 +2,7 @@ import { useEffect, useLayoutEffect, useMemo, useRef, useState } from "react";
 import { motion } from "framer-motion";
 import { convertFileSrc } from "@tauri-apps/api/core";
 import { ExploreTagEntry, TagCloudEntry, useGalleryStore } from "../store";
+import { FolderScopeDropdown } from "./FolderScopeDropdown";
 
 const ACCENTS = [
   "#60a5fa",
@@ -317,23 +318,26 @@ export function TagCloud() {
                     : "No tags — run the AI tagger or add tags manually"}
             </p>
           </div>
-          <div className="flex shrink-0 rounded-lg border border-white/8 bg-white/[0.03] p-0.5">
-            <button
-              className={`rounded-md px-3 py-1.5 text-xs transition-colors ${
-                exploreMode === "visual" ? "bg-white/10 text-white" : "text-gray-500 hover:text-gray-300"
-              }`}
-              onClick={() => setExploreMode("visual")}
-            >
-              Clusters
-            </button>
-            <button
-              className={`rounded-md px-3 py-1.5 text-xs transition-colors ${
-                exploreMode === "tags" ? "bg-white/10 text-white" : "text-gray-500 hover:text-gray-300"
-              }`}
-              onClick={() => setExploreMode("tags")}
-            >
-              Tag Cloud
-            </button>
+          <div className="flex shrink-0 items-center gap-2">
+            <FolderScopeDropdown />
+            <div className="flex rounded-lg border border-white/8 bg-white/[0.03] p-0.5">
+              <button
+                className={`rounded-md px-3 py-1.5 text-xs transition-colors ${
+                  exploreMode === "visual" ? "bg-white/10 text-white" : "text-gray-500 hover:text-gray-300"
+                }`}
+                onClick={() => setExploreMode("visual")}
+              >
+                Clusters
+              </button>
+              <button
+                className={`rounded-md px-3 py-1.5 text-xs transition-colors ${
+                  exploreMode === "tags" ? "bg-white/10 text-white" : "text-gray-500 hover:text-gray-300"
+                }`}
+                onClick={() => setExploreMode("tags")}
+              >
+                Tag Cloud
+              </button>
+            </div>
           </div>
         </div>
       </div>