feat(v0.2): tab title = current folder (from OSC title)

- title-util.ts: folderFromTitle() parses 'user@host:~/path' → last folder
  segment (home stays ~); pure, dependency-free, 7 unit tests
- terminal-session.ts: onTitleChange → folderFromTitle so the tab auto-names
  to the cwd folder and updates live on cd
- Browser-verified: ~ → cd project → 'web-terminal' → cd docs → 'docs' → cd ~ → '~'.
  198 tests green, typecheck + build ok.

test/title-util.test.ts

@@ -0,0 +1,33 @@
+import { describe, it, expect } from 'vitest'
+import { folderFromTitle } from '../public/title-util.js'
+
+describe('folderFromTitle', () => {
+  it('extracts the folder from user@host:~/path', () => {
+    expect(folderFromTitle('yiukai@Mac:~/Documents/git/web-terminal')).toBe('web-terminal')
+  })
+
+  it('keeps ~ for the home directory', () => {
+    expect(folderFromTitle('yiukai@Yiukais-MacBook-Pro:~')).toBe('~')
+  })
+
+  it('takes the basename of an absolute path', () => {
+    expect(folderFromTitle('user@host:/Users/yiukai/web')).toBe('web')
+  })
+
+  it('strips a trailing slash', () => {
+    expect(folderFromTitle('user@host:~/web/')).toBe('web')
+  })
+
+  it('handles a title with no user@host: prefix', () => {
+    expect(folderFromTitle('web-terminal')).toBe('web-terminal')
+  })
+
+  it('returns the last path segment when the whole title is a path', () => {
+    expect(folderFromTitle('/var/log/nginx')).toBe('nginx')
+  })
+
+  it('returns null for an empty/whitespace title', () => {
+    expect(folderFromTitle('   ')).toBeNull()
+    expect(folderFromTitle('')).toBeNull()
+  })
+})