polish(v0.6): replace toolbar emoji with themed line icons + hover tooltips

The glossy emoji (🔍⚙▦🕘⌨🔗📱) clashed with the Amber dark theme. Swap them
for clean lucide line icons (MIT, stroke=currentColor) so they take the theme
colour — muted by default, Amber on hover — and add a styled CSS hover tooltip
(data-tip, right-anchored so it never clips the edge) showing what each does.

- public/icons.ts: 7 line icons (search/settings/dashboard/history/keyboard/
  share/device), script-generated from lucide
- 7 toolbar modules: innerHTML = ICON_* + data-tip + aria-label
- style.css .toolbtn: 18px svg, --text-dim → --accent on hover, [data-tip] tooltip

Frontend-only. web typecheck + build clean; bundle has 0 toolbar emoji; verified
in-browser: line icons in theme colour, amber + tooltip on hover.

public/shortcuts.ts

@@ -6,6 +6,8 @@
  * are tagged "键栏". Source: official Claude Code interactive-mode docs.
  */
 
+import { ICON_KEYBOARD } from './icons.js'
+
 interface Shortcut {
   keys: string
   desc: string
@@ -132,8 +134,9 @@ export function mountShortcuts(toolbar: HTMLElement): void {
 
   const toggle = document.createElement('button')
   toggle.className = 'toolbtn'
-  toggle.textContent = '⌨'
-  toggle.title = 'Claude Code 快捷键速查'
+  toggle.innerHTML = ICON_KEYBOARD
+  toggle.dataset.tip = 'Keyboard shortcuts'
+  toggle.setAttribute('aria-label', 'Keyboard shortcuts')
   toggle.setAttribute('aria-label', 'Keyboard shortcuts')
   toggle.addEventListener('click', () => {
     overlay.style.display = overlay.style.display === 'none' ? 'flex' : 'none'