Fix login button and improve CORS

- Fixed login URL from /auth/steam to /api/auth/steam
- Updated all Steam login buttons to custom green design with 'Login to Steam' text
- Enhanced CORS configuration with explicit preflight handling
- Added Steam image proxy endpoint for CORS-free image loading
- Improved environment variable management with .env.local support
- Added ENV_SETUP.md guide for environment configuration

frontend/src/components/NavBar.vue

@@ -250,12 +250,17 @@ onUnmounted(() => {
           </div>
 
           <!-- Login Button -->
-          <button v-else @click="handleLogin" class="btn btn-primary">
-            <img
-              src="https://community.cloudflare.steamstatic.com/public/images/signinthroughsteam/sits_01.png"
-              alt="Sign in through Steam"
-              class="h-6"
-            />
+          <button
+            v-else
+            @click="handleLogin"
+            class="px-6 py-2.5 bg-gradient-to-r from-green-600 to-green-700 hover:from-green-700 hover:to-green-800 text-white font-semibold rounded-lg transition-all duration-200 shadow-lg hover:shadow-xl flex items-center gap-2"
+          >
+            <svg class="w-5 h-5" viewBox="0 0 24 24" fill="currentColor">
+              <path
+                d="M12 2a10 10 0 0 0-10 10 10 10 0 0 0 10 10c.5 0 1-.04 1.48-.1L8.44 14.3a3.2 3.2 0 0 1-.94-2.27c0-1.77 1.43-3.2 3.2-3.2.53 0 1.03.13 1.47.36L15.5 6.7A9.96 9.96 0 0 0 12 2m6.5 4.5-4.67 2.13c.52.47.84 1.15.84 1.9 0 1.41-1.15 2.57-2.57 2.57-.17 0-.33-.02-.49-.05l-2.25 3.25.02.02c2.25.55 4.77-.51 6.41-2.8 2.02-2.83 1.78-6.7-.29-7.02z"
+              />
+            </svg>
+            Login to Steam
           </button>
 
           <!-- Mobile Menu Toggle -->