feat: keybar function captions + Ctrl+R/L/D, plus ⌨ shortcut cheat-sheet

keybar:
- add Ctrl+R (搜历史/\x12), Ctrl+L (重绘/\x0c), Ctrl+D (退出/\x04)
- each key now shows a short function caption under the glyph (self-documenting
  on touch where tooltips don't appear); 17 keys total
- test/keybar.test.ts updated to 17 keys

cheat-sheet (new public/shortcuts.ts):
- ⌨ toolbar button opens a grouped reference of common Claude Code shortcuts
  (general / editing / multiline / macOS / prefixes), each labeled with what it
  does; keys also on the bottom bar are tagged 键栏
- Esc / click-outside to close; styled card with sticky header + accent tags

216 tests green; tsc + build clean.

public/main.ts

@@ -18,6 +18,7 @@ import { mountQrConnect } from './qr.js'
 import { mountSettings, loadSettings, type Settings } from './settings.js'
 import { mountDashboard } from './dashboard.js'
 import { mountHistory } from './history.js'
+import { mountShortcuts } from './shortcuts.js'
 
 const paneHost = document.getElementById('term')
 const tabs = document.getElementById('tabs')
@@ -55,6 +56,7 @@ mountDashboard(toolbar, {
 mountHistory(toolbar, {
   resume: (cwd, id) => app.newTabForResume(cwd, id),
 })
+mountShortcuts(toolbar)
 mountQrConnect(toolbar)
 
 // PWA: register the service worker (installable + offline shell, M4).