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/settings.ts

@@ -6,6 +6,7 @@
  */
 
 import type { ITheme } from '@xterm/xterm'
+import { ICON_SETTINGS } from './icons.js'
 
 export interface Settings {
   theme: string
@@ -118,8 +119,9 @@ export function mountSettings(
 
   const toggle = document.createElement('button')
   toggle.className = 'toolbtn'
-  toggle.textContent = '⚙'
-  toggle.title = 'Settings (theme, font)'
+  toggle.innerHTML = ICON_SETTINGS
+  toggle.dataset.tip = 'Settings'
+  toggle.setAttribute('aria-label', 'Settings')
   toggle.setAttribute('aria-label', 'Settings')
   toggle.addEventListener('click', () => {
     if (panel.style.display === 'none') {