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/push.ts

@@ -319,9 +319,9 @@ function renderToggleButton(
   )
   btn.innerHTML = ICON_BELL
   btn.append(' ' + (isSubscribed ? 'On' : 'Off'))
-  btn.title = isSubscribed
-    ? 'Click to disable push notifications'
-    : 'Click to enable push notifications'
+  btn.dataset.tip = isSubscribed
+    ? 'Notifications on — tap to turn off'
+    : 'Notifications off — tap to turn on'
   btn.setAttribute('aria-pressed', isSubscribed ? 'true' : 'false')
 
   btn.addEventListener('click', () => {