feat(notifications): add opt-in timeout progress bar on popups (#2587)

Adds a thin bar pinned to the bottom of the notification card that drains
full->empty over the auto-dismiss timer, as a visual countdown to
dismissal. Opt-in via notificationShowTimeoutBar (default off), with a
toggle in Settings > Notifications. Shown for any timed notification
(timer.interval > 0, including timed criticals); inset by the corner
radius, and frozen while hovered or during the exit animation. Plain
Rectangle - no offscreen textures or shader passes. A Connections on the
timer resets the bar on every (re)start, including the in-place restart
on a deduped notification.

Co-authored-by: bogdan-velicu <hydrotech074@gmail.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

quickshell/Common/settings/SettingsSpec.js

@@ -406,6 +406,7 @@ var SPEC = {
     notificationTimeoutNormal: { def: 5000 },
     notificationTimeoutCritical: { def: 0 },
     notificationCompactMode: { def: false },
+    notificationShowTimeoutBar: { def: false },
     notificationDedupeEnabled: { def: true },
     notificationPopupPosition: { def: 0 },
     notificationAnimationSpeed: { def: 1 },