Add the Sandbar mobile companion PWA

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

public/manifest.webmanifest

@@ -0,0 +1,16 @@
+{
+  "name": "Sandbar",
+  "short_name": "Sandbar",
+  "description": "Alerts, on-call and monitors from your Sandbar server.",
+  "start_url": "./",
+  "scope": "./",
+  "display": "standalone",
+  "background_color": "#101515",
+  "theme_color": "#1f5c5c",
+  "icons": [
+    { "src": "icon-192.png", "sizes": "192x192", "type": "image/png", "purpose": "any" },
+    { "src": "icon-512.png", "sizes": "512x512", "type": "image/png", "purpose": "any" },
+    { "src": "icon-512.png", "sizes": "512x512", "type": "image/png", "purpose": "maskable" },
+    { "src": "icon.svg", "sizes": "any", "type": "image/svg+xml" }
+  ]
+}