## What happened

Critical-urgency notifications (`--urgency critical` / `NotificationUrgency.Critical`) have no visible way to dismiss them and never auto-expire, so they can get stuck on screen indefinitely.

This is most visible with the built-in crash-detection feature: `omarchy-crash-watch` sends its "Process crashed: X — Click to diagnose with AI" toast with `--urgency critical`. If the user doesn't already know that **right-click** dismisses a notification card (there is no visible close button, no `X`, nothing in the UI hints at it), that toast just sits there permanently, since `Service.qml`'s `durationFor()` explicitly returns `0` (no timer) for `NotificationUrgency.Critical`.

Reproduce directly, no crash needed:

```bash
omarchy notification send -u critical "Test" "This will never go away and has no visible close button"
```

## What was expected

Either:
- a small, always-visible dismiss button on the notification card (in addition to right-click), and/or
- critical notifications eventually auto-expiring after a generous timeout (e.g. 20-30s) instead of never, since "never expires" + "no visible way to close" compounds into a notification that's effectively stuck forever for anyone who hasn't read the source.

## Suggested fix

In `/usr/share/omarchy/shell/plugins/notifications/components/NotificationCard.qml`, add a small `×` button (top-right corner of the card) wired to the existing `closeRequested()` signal — the plumbing already exists (right-click already calls it), it's just not exposed visually.

In `/usr/share/omarchy/shell/plugins/notifications/Service.qml`, `durationFor()` could give `NotificationUrgency.Critical` a bounded duration (e.g. `Math.min(maxPopupDuration, Math.max(20000, requestedDuration(expireTimeout)))`) instead of hardcoding `0`. The per-card countdown plumbing (`cardSlot.lifetime`, `remainingLifetime`, the hover-pauses-countdown `Timer`) already exists and already handles `lifetime <= 0` as "never expire" — enabling it for critical is a one-line change.

I patched a local clone of the `notifications` plugin with exactly this (visible close button + bounded critical timeout + a small depleting progress bar reusing the already-computed `remainingLifetime` value) and it works well; happy to open a PR with it if that's useful.

## Environment

- Omarchy version: 4.0.1-1
- Kernel: `7.1.9-arch1-2` (Arch Linux)
- GPU: Intel UHD Graphics 620 (i915)
- Compositor: Hyprland (default Omarchy shell / Quickshell)

(`omarchy debug --no-sudo --print` is not available as a command on this installed version, so I couldn't attach the standard diagnostic log — `omarchy commands --all` doesn't list a `debug` command either.)