fix: brainstorm infinite loop, ghost cursor, embedding ::vector cast, semantic search, billing stats, usage meter accordion

- Fix useBrainstormSocket: stable guestId via useRef, remove setState in cleanup
- Fix GhostCursor: direct DOM manipulation via refs, no useState re-renders
- Fix all SQL embedding queries: add ::vector cast on text columns
- Fix embedding truncation to 15000 chars (under 8192 token limit)
- Fix NoteEmbedding INSERT: remove non-existent updatedAt column
- Fix billing page: show all quota stats in grid instead of single metric
- Fix usage meter: accordion expand/collapse, per-feature detail
- Fix semantic search: rebuild 103 note embeddings, ::vector cast on vectorSearch
- Fix brainstorm expand/manual-idea/create: ::vector cast on embedding SQL

memento-note/components/hierarchical-notebook-selector.tsx

@@ -40,7 +40,10 @@ export function HierarchicalNotebookSelector({
   const getDropdownStyle = useCallback((): React.CSSProperties => {
     if (!triggerRef.current) return { position: 'fixed', top: 0, left: 0, width: 280, zIndex: 9999 }
     const rect = triggerRef.current.getBoundingClientRect()
-    if (dropUp) {
+    const dropdownHeight = 350
+    const viewportHeight = window.innerHeight
+    const wouldOverflowBottom = rect.bottom + 8 + dropdownHeight > viewportHeight
+    if (dropUp || wouldOverflowBottom) {
       return {
         position: 'fixed',
         bottom: window.innerHeight - rect.top + 8,