refactor: extract frontmatterOps, update imports for useNoteCreation/useNoteRename split

useNoteActions.ts reduced from 213 to 125 lines by extracting frontmatter
helpers into frontmatterOps.ts and removing re-exports. Consumers now
import directly from the extracted modules.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/components/TitleField.tsx

@@ -1,5 +1,5 @@
 import { useState, useCallback, useRef, useEffect } from 'react'
-import { slugify } from '../hooks/useNoteActions'
+import { slugify } from '../hooks/useNoteCreation'
 
 interface TitleFieldProps {
   title: string