polish(v0.7): styled hover tooltips on timeline + push toggle (explain the icon)

Give the two tab-bar line-icon buttons the same styled data-tip hover tooltip as
the toolbar icons, so their meaning is clear on hover (native title was slow/ugly):
- timeline → "Activity timeline"
- push toggle → "Notifications off — tap to turn on" / "…on — tap to turn off"
Left-anchored (they sit on the left of the tab bar). 158 push+tabs tests green.

public/tabs.ts

@@ -855,7 +855,7 @@ export class TabApp {
     const tl = document.createElement('button')
     tl.className = 'tab-timeline'
     tl.innerHTML = ICON_TIMELINE
-    tl.title = 'Activity timeline'
+    tl.dataset.tip = 'Activity timeline'
     tl.setAttribute('aria-label', 'Activity timeline')
     tl.classList.toggle('active', this.timelineOpen)
     tl.addEventListener('click', () => this.toggleTimeline())