fix: prevent crash in handleCreateNoteImmediate — slugify fallback + try/catch

- slugify now returns 'untitled' instead of empty string when input has only
  special characters, preventing invalid paths like '/vault//note.md'
- handleCreateNoteImmediate wrapped in try/catch — worst case shows a toast
  error instead of crashing the app
- Added Rust test for deeply nested directory creation in save_note_content
- Regression tests for slugify edge cases and handleCreateNoteImmediate with
  special-character types

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

src/hooks/useNoteActions.test.ts

@@ -77,13 +77,21 @@ describe('slugify', () => {
     expect(slugify('--hello--')).toBe('hello')
   })
 
-  it('handles empty string', () => {
-    expect(slugify('')).toBe('')
+  it('handles empty string with fallback', () => {
+    expect(slugify('')).toBe('untitled')
   })
 
   it('collapses multiple separators into one hyphen', () => {
     expect(slugify('hello   world---foo')).toBe('hello-world-foo')
   })
+
+  it('returns fallback for strings with only special characters', () => {
+    // slugify('+++') should not return empty string — it causes invalid paths
+    expect(slugify('+++')).not.toBe('')
+    expect(slugify('!!!')).not.toBe('')
+    expect(slugify('---')).not.toBe('')
+    expect(slugify('@#$')).not.toBe('')
+  })
 })
 
 describe('buildNewEntry', () => {
@@ -270,6 +278,20 @@ describe('resolveNewNote', () => {
     expect(entry.path).toBe('/other/vault/note/test.md')
     expect(entry.path).not.toContain('/Users/luca/Laputa')
   })
+
+  it('produces a valid path for custom types with special characters', () => {
+    const { entry } = resolveNewNote('My Note', 'Q&A', '/vault')
+    expect(entry.path).not.toContain('//')
+    expect(entry.path).toMatch(/\.md$/)
+    expect(entry.filename).not.toBe('.md')
+  })
+
+  it('produces a valid path when type is all special characters', () => {
+    const { entry } = resolveNewNote('My Note', '+++', '/vault')
+    // folder should not be empty, path should not have double slashes
+    expect(entry.path).not.toContain('//')
+    expect(entry.path).toMatch(/\.md$/)
+  })
 })
 
 describe('resolveNewType', () => {
@@ -612,6 +634,34 @@ describe('useNoteActions hook', () => {
     expect(tabContent).toContain('## Steps')
   })
 
+  it('handleCreateNoteImmediate does not throw for custom types with special characters', () => {
+    const { result } = renderHook(() => useNoteActions(makeConfig()))
+
+    expect(() => {
+      act(() => {
+        result.current.handleCreateNoteImmediate('Q&A')
+      })
+    }).not.toThrow()
+
+    const [entry] = addEntry.mock.calls[0]
+    expect(entry.isA).toBe('Q&A')
+    expect(entry.path).not.toContain('//')
+  })
+
+  it('handleCreateNoteImmediate does not throw for types that slugify to empty string', () => {
+    const { result } = renderHook(() => useNoteActions(makeConfig()))
+
+    expect(() => {
+      act(() => {
+        result.current.handleCreateNoteImmediate('+++')
+      })
+    }).not.toThrow()
+
+    const [entry] = addEntry.mock.calls[0]
+    expect(entry.path).not.toContain('//')
+    expect(entry.filename).not.toBe('.md')
+  })
+
   it('handleCreateNoteImmediate uses template for typed notes', () => {
     const typeEntry = makeEntry({ isA: 'Type', title: 'Project', template: '## Custom Template\n\n' })
     const { result } = renderHook(() => useNoteActions(makeConfig([typeEntry])))