Improve suggested notes: limit 8, threshold 0.45, show relevance scores

- Raise similarity threshold 0.30 → 0.45: only genuinely relevant notes
  shown; loosely-related notes no longer pad the sidebar
- Increase max suggested notes 3 → 8 (zero added compute — threshold is
  the real gate; the embedding call is fixed regardless of limit)
- semantic_search_notes now returns list[tuple[float, Note]] instead of
  list[Note] so scores propagate through context_meta to the frontend
- Keyword fallback notes carry score=null (no cosine similarity available)
- ChatView sidebar shows % badge on each suggested note:
  green ≥75%, amber 60–74%, muted <60%
  Hovering reveals the raw score in a tooltip

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/types/chat.ts

@@ -54,7 +54,7 @@ export interface ConversationDetail extends Conversation {
 export interface ContextMeta {
   context_note_id: number | null;
   context_note_title: string | null;
-  auto_notes: { id: number; title: string }[];
+  auto_notes: { id: number; title: string; score?: number | null }[];
 }
 
 export interface OllamaStatus {