feat: PWA setup + /app base path

- Landing page at /
- Vue app at /app (PWA installable)
- Added manifest.json for mobile install
- iOS + Android meta tags
- Updated router base path

public/favicon.svg

@@ -0,0 +1,10 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100">
+  <defs>
+    <linearGradient id="grad" x1="0%" y1="0%" x2="100%" y2="100%">
+      <stop offset="0%" style="stop-color:#667eea"/>
+      <stop offset="100%" style="stop-color:#764ba2"/>
+    </linearGradient>
+  </defs>
+  <rect width="100" height="100" rx="20" fill="url(#grad)"/>
+  <text x="50" y="68" font-size="50" text-anchor="middle" fill="white">🔐</text>
+</svg>