feat: folder navigation from sidebar

Folders in the LeftSidebar were decorative — clicking one did
nothing. Now they actually filter the timeline.

filterStore: new folderId field, setFolderId, hasActiveFilters check,
filtersToParams sends folder_id to the backend (the param was already
declared and applied server-side, just nothing was setting it).
useFilterUrlSync round-trips ?folder_id= so the filter persists in
the URL. usePhotosQuery threads it through.

LeftSidebar:
- Clicking a folder row calls clearAllFilters() then setFolderId(id)
  so the user lands cleanly on that folder.
- Library virtual nodes (All Photos, Rated, Discarded) clear the
  folder filter as part of their normal action.
- The active-row visual highlight is now derived from the filter
  store: a folder row is selected when filterStore.folderId matches
  it, "All Photos" is selected when no folder is set. Keeps the
  sidebar in sync if filters change externally (URL hydrate, the
  ActiveFilterChips X button, FilterBar Clear all).

ActiveFilterChips: shows "Folder: {name}" and "Heap: {name}" chips,
looking up the names from the folders / heaps queries (lazy-enabled
only when the corresponding filter is set). Clicking the X clears
the filter.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/components/layout/LeftSidebar.tsx

@@ -39,6 +39,8 @@ export function LeftSidebar() {
   const clearAllFilters = useFilterStore((s) => s.clearAll)
   const setRatingMin = useFilterStore((s) => s.setRatingMin)
   const setFlag = useFilterStore((s) => s.setFlag)
+  const setFolderId = useFilterStore((s) => s.setFolderId)
+  const filterFolderId = useFilterStore((s) => s.folderId)
 
   // Map a library tree id to a filter-store mutation. Each "virtual node" in
   // the library tree is just a saved filter preset.
@@ -55,7 +57,16 @@ export function LeftSidebar() {
         clearAllFilters()
         setFlag('discarded')
         break
-      // 'by-date' is purely visual until we add a date-grouping UI
+      default:
+        if (id.startsWith('folder-')) {
+          // Folder rows: filter to that folder, clear other filters that
+          // would compete (heap, discarded, etc.) so the user sees what they
+          // expect when they click a folder.
+          const folderId = id.slice('folder-'.length)
+          clearAllFilters()
+          setFolderId(folderId)
+        }
+        // 'by-date' is still visual-only.
     }
   }
   
@@ -150,10 +161,24 @@ export function LeftSidebar() {
     },
   ]
   
+  // Derive whether a tree item is currently the "active" filter target.
+  // Folder rows are selected when the filter store's folderId matches; the
+  // library "All Photos" virtual node is selected when no folder/heap filter
+  // is set.
+  const isItemActive = (id: string): boolean => {
+    if (id.startsWith('folder-')) {
+      return filterFolderId === id.slice('folder-'.length)
+    }
+    if (id === 'all-photos') {
+      return filterFolderId === null && selectedItem === 'all-photos'
+    }
+    return selectedItem === id
+  }
+
   const renderTreeItem = (item: TreeItem, depth: number = 0) => {
     const hasChildren = item.children && item.children.length > 0
     const isExpanded = expandedItems.has(item.id)
-    const isSelected = selectedItem === item.id
+    const isSelected = isItemActive(item.id)
     
     return (
       <div key={item.id}>