feat: distinguish new notes (green dot) from modified notes (orange dot)

New notes created in-session show a green dot; existing notes with
uncommitted git changes show an orange dot. Saving a new note clears
the green dot; git commit clears the orange dot.

Introduces NoteStatus type ('new' | 'modified' | 'clean'), extracts
useNewNoteTracker hook and resolveNoteStatus pure function, and adds
onNoteSaved callback to useEditorSave for clearing new-note tracking.

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

src/types.ts

@@ -26,6 +26,8 @@ export interface VaultEntry {
   order: number | null
 }
 
+export type NoteStatus = 'new' | 'modified' | 'clean'
+
 export interface GitCommit {
   hash: string
   shortHash: string