[FEATURE] Add mirror search/selection

Summary: Add mirror search/selection

Mirror search and selection

### Problem

`pacman-mirrorlist` and manual editing are clunky; users want faster mirror pickers aligned with Pacsea’s UX.

### Recommended implementation

- **Data source:** `reflector` if present (optional dependency), else parse public mirror JSON/CSV if already bundled or fetched with documented TTL cache.
- **UI:** Modal or dedicated panel: filter by country, protocol, HTTPS, sort by score/latency from reflector output.
- **Apply path:** Write to `/etc/pacman.d/mirrorlist` only through privileged command builder (sudo/doas) with confirmation and `--dry-run` preview of file diff.
- **Safety:** Backup previous mirrorlist to timestamped path or rely on pacsave pattern; document restore steps in error messages.

### Acceptance criteria

- Graceful degradation when `reflector` absent (suggest install).
- Dry-run shows exact command and target paths without writing.

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