feat: rename file on save when title slug doesn't match filename

When Cmd+S is pressed, after saving content, checks if the note's
title slug differs from its current filename. If so, triggers
rename_note to update the file on disk, tabs, breadcrumbs, and
wikilinks. Adds needsRenameOnSave() utility with tests.

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

src/hooks/useNoteActions.test.ts

@@ -5,6 +5,7 @@ import { isTauri, mockInvoke } from '../mock-tauri'
 import type { VaultEntry } from '../types'
 import {
   slugify,
+  needsRenameOnSave,
   buildNewEntry,
   generateUntitledName,
   entryMatchesTarget,
@@ -94,6 +95,22 @@ describe('slugify', () => {
   })
 })
 
+describe('needsRenameOnSave', () => {
+  it('returns true when filename does not match title slug', () => {
+    expect(needsRenameOnSave('My New Note', 'untitled-note.md')).toBe(true)
+    expect(needsRenameOnSave('Run good ads for newsletter', 'untitled-note-9.md')).toBe(true)
+  })
+
+  it('returns false when filename matches title slug', () => {
+    expect(needsRenameOnSave('My Note', 'my-note.md')).toBe(false)
+    expect(needsRenameOnSave('Hello World', 'hello-world.md')).toBe(false)
+  })
+
+  it('returns false for untitled note with matching slug', () => {
+    expect(needsRenameOnSave('Untitled note', 'untitled-note.md')).toBe(false)
+  })
+})
+
 describe('buildNewEntry', () => {
   it('creates a VaultEntry with correct fields', () => {
     const entry = buildNewEntry({