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/app/(public)/layout.tsx

@@ -0,0 +1,17 @@
+import { headers } from 'next/headers'
+import { detectUserLanguage, parseAcceptLanguage } from '@/lib/i18n/detect-user-language'
+import { loadTranslations } from '@/lib/i18n/load-translations'
+import { PublicProviders } from '@/components/public-providers'
+
+export default async function PublicLayout({ children }: { children: React.ReactNode }) {
+  const headersList = await headers()
+  const browserLang = parseAcceptLanguage(headersList.get('accept-language'))
+  const initialLanguage = await detectUserLanguage(browserLang)
+  const initialTranslations = await loadTranslations(initialLanguage)
+
+  return (
+    <PublicProviders initialLanguage={initialLanguage} initialTranslations={initialTranslations}>
+      {children}
+    </PublicProviders>
+  )
+}

memento-note/app/(public)/page.tsx

@@ -0,0 +1,5 @@
+import { LandingPage } from '@/components/landing-page'
+
+export default function PublicHomePage() {
+  return <LandingPage />
+}