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/lib/ai/services/paragraph-refactor.service.ts

@@ -286,11 +286,15 @@ Only return the translated text, nothing else.`,
 Keep the explanation clear, educational, and concise.`
     }
 
+    const systemSuffix = mode === 'explain'
+      ? `CRITICAL: Respond ONLY with your explanation in ${language}. No meta-commentary, no English. Format strictly as ${format === 'html' ? 'HTML tags' : 'Markdown'}.`
+      : `CRITICAL: Respond ONLY with the refactored text in ${language}. No explanations, no meta-commentary, no English. Format strictly as ${format === 'html' ? 'HTML tags' : 'Markdown'}.`
+
     return `${instructions[mode]}
 
 ${content}
 
-CRITICAL: Respond ONLY with the refactored text in ${language}. No explanations, no meta-commentary, no English. Format strictly as ${format === 'html' ? 'HTML tags' : 'Markdown'}.`
+${systemSuffix}`
   }
 
   /**