polish(v0.7): replace timeline 📜 + push 🔔 emoji with themed line icons

Swap the two v0.7 emoji buttons for lucide line icons (stroke=currentColor) so
they take the Amber theme colour like the toolbar icons:
- timeline toggle (tabs.ts) → scroll-text icon
- push toggle + disabled bell (push.ts) → bell icon, keeping the On/Off label
- icons.ts: ICON_TIMELINE, ICON_BELL; style.css sizes the inline svgs (16px)

Frontend-only. web tsc + build:web clean, 102 push tests green, bundle emoji-free.

public/tabs.ts

@@ -22,6 +22,7 @@
  */
 
 import { TerminalSession } from './terminal-session.js'
+import { ICON_TIMELINE } from './icons.js'
 import { THEMES, DEFAULT_SETTINGS, type Settings } from './settings.js'
 import { mountLauncher, type Launcher } from './launcher.js'
 import { mountProjects, type ProjectsPanel } from './projects.js'
@@ -853,7 +854,7 @@ export class TabApp {
     // survive tabBar.replaceChildren() (the bell's subtree is mounted once).
     const tl = document.createElement('button')
     tl.className = 'tab-timeline'
-    tl.textContent = '📜'
+    tl.innerHTML = ICON_TIMELINE
     tl.title = 'Activity timeline'
     tl.setAttribute('aria-label', 'Activity timeline')
     tl.classList.toggle('active', this.timelineOpen)