feat(timeline): add virtualised month-grouped timeline view

- New Timeline view groups all media by YYYY-MM using taken_at ?? created_at
- @tanstack/react-virtual (group-level virtualiser) keeps scroll smooth on
  large libraries; estimateSize is exact so no measureElement needed
- ResizeObserver tracks container width; virtualizer.measure() is called on
  cols change to prevent stale position cache after window resize
- setView("timeline") in store auto-sets sort:"taken_asc", resets images,
  and triggers a fresh load
- Calendar nav item added to Sidebar between Explore and Duplicates
- ContextMenu and ImageTile exported from Gallery for reuse in Timeline

src/components/Sidebar.tsx

@@ -329,6 +329,23 @@ export function Sidebar() {
           </span>
         </div>
 
+        <div
+          className={`flex items-center gap-2.5 px-3 py-2 rounded-lg cursor-pointer transition-all duration-150 ${
+            activeView === "timeline"
+              ? "bg-white/8 text-white"
+              : "text-gray-500 hover:text-gray-200 hover:bg-white/5"
+          }`}
+          onClick={() => setView("timeline")}
+        >
+          <svg className="w-3.5 h-3.5 shrink-0" fill="none" viewBox="0 0 24 24" stroke="currentColor">
+            <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={1.5}
+              d="M8 7V3m8 4V3m-9 8h10M5 21h14a2 2 0 002-2V7a2 2 0 00-2-2H5a2 2 0 00-2 2v12a2 2 0 002 2z" />
+          </svg>
+          <span className={`text-[13px] font-medium ${activeView === "timeline" ? "text-white" : ""}`}>
+            Timeline
+          </span>
+        </div>
+
         <div
           className={`flex items-center gap-2.5 px-3 py-2 rounded-lg cursor-pointer transition-all duration-150 ${
             activeView === "duplicates"