release(v1.7.28-alpha): reboot progress overlay + VPS default primary

- New reboot progress overlay: full-screen black with the screensaver's
  pulsing ring, rebooting → reconnecting → back-online → stalled stages,
  elapsed counter, auto-reload on health-check success, manual reload
  button at 3 min stall. Mirrors the existing update overlay.
- Ring extracted from Screensaver.vue into a reusable ScreensaverRing
  component so the reboot overlay reuses the same animation.
- default_mirrors() now puts the VPS as Server 1 (primary) and tx1138 as
  Server 2 — new nodes fetch manifests from VPS first; existing nodes
  keep whatever mirror order they've customized.
- What's New entry prepended for v1.7.28-alpha.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

core/archipelago/src/update.rs

@@ -90,12 +90,12 @@ fn mirrors_path(data_dir: &Path) -> std::path::PathBuf {
 fn default_mirrors() -> Vec<UpdateMirror> {
     vec![
         UpdateMirror {
-            url: DEFAULT_UPDATE_MANIFEST_URL.to_string(),
-            label: "Server 1 (tx1138)".to_string(),
+            url: DEFAULT_SECONDARY_MIRROR_URL.to_string(),
+            label: "Server 1 (VPS)".to_string(),
         },
         UpdateMirror {
-            url: DEFAULT_SECONDARY_MIRROR_URL.to_string(),
-            label: "Server 2 (VPS)".to_string(),
+            url: DEFAULT_UPDATE_MANIFEST_URL.to_string(),
+            label: "Server 2 (tx1138)".to_string(),
         },
     ]
 }