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/Components/Layout/NavMenu.razor

@@ -2,7 +2,7 @@
 
 <div class="nav-header">
     <a class="nav-brand" href="">
-        <span class="nav-brand-icon">🐢</span>
+        <img src="logo.png" alt="GM-Relay" class="nav-brand-icon" />
         <span class="nav-brand-text">GM-Relay</span>
     </a>
     <button class="nav-toggle" @onclick="ToggleMenu" aria-label="Переключить меню">