feat(frontend): description field in task editor + Goal block in viewer

Note type gains description and consolidated_at fields. TaskEditorView
adds a Goal textarea above the body editor (wired through dirty/save/
autosave paths). TaskViewerView renders Goal as a subordinate block
above the body, plus a subtle 'Auto-summarized from work logs' banner
when consolidated_at is set.

Also adds a consolidateTask client function for the upcoming
re-consolidate button (Task 11).

frontend/src/types/note.ts

@@ -6,6 +6,8 @@ export interface Note {
   id: number;
   title: string;
   body: string;
+  description: string | null;
+  consolidated_at: string | null;
   tags: string[];
   parent_id: number | null;
   parent_title?: string | null;