feat(ui): replace emoji logos with new app icon across dashboard

- NavMenu: swap 🐢 emoji for <img src="logo.png">
- Login page: swap 🎲 emoji for <img src="logo.png">
- Mini App page: swap 🎲 emoji for <img src="logo.png">
- Replace favicon.png with the new logo
- Add logo.png to wwwroot
- Update CSS for .nav-brand-icon, .login-logo, .mini-app-logo to use object-fit: contain sizing

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/GmRelay.Web/wwwroot/app.css

@@ -877,8 +877,13 @@ select option {
 }
 
 .login-logo {
-    font-size: 2.5rem;
+    width: 2.5rem;
+    height: 2.5rem;
+    object-fit: contain;
     margin-bottom: 0.5rem;
+    display: block;
+    margin-left: auto;
+    margin-right: auto;
 }
 
 .login-title {
@@ -919,8 +924,13 @@ select option {
 }
 
 .mini-app-logo {
-    font-size: 2.25rem;
+    width: 2.25rem;
+    height: 2.25rem;
+    object-fit: contain;
     margin-bottom: 0.75rem;
+    display: block;
+    margin-left: auto;
+    margin-right: auto;
 }
 
 .mini-app-auth-card h1 {