feat: add Steam OpenID login

- Add Steam OpenID 2.0 authentication routes (login + callback)
- Enable Steam button in LoginModal (was placeholder)
- Unified user ID system: getUserId() supports Discord, Steam, Admin
- Update soundboard user-sound endpoints for Steam users
- UserSettings now works for both Discord and Steam providers
- Steam hover uses brand color #66c0f4

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/src/App.tsx

@@ -17,8 +17,9 @@ interface PluginInfo {
 
 interface AuthUser {
   authenticated: boolean;
-  provider?: 'discord' | 'admin';
+  provider?: 'discord' | 'steam' | 'admin';
   discordId?: string;
+  steamId?: string;
   username?: string;
   avatar?: string | null;
   globalName?: string | null;
@@ -69,6 +70,8 @@ export default function App() {
   // Derived state
   const isAdmin = user.authenticated && (user.provider === 'admin' || user.isAdmin === true);
   const isDiscordUser = user.authenticated && user.provider === 'discord';
+  const isSteamUser = user.authenticated && user.provider === 'steam';
+  const isRegularUser = isDiscordUser || isSteamUser;
 
   // Electron auto-update state
   const isElectron = !!(window as any).electronAPI?.isElectron;
@@ -239,7 +242,7 @@ export default function App() {
       setShowLoginModal(true);
     } else if (isAdmin) {
       setShowAdminPanel(true);
-    } else if (isDiscordUser) {
+    } else if (isRegularUser) {
       setShowUserSettings(true);
     }
   }
@@ -294,7 +297,7 @@ export default function App() {
             }
           >
             {user.authenticated ? (
-              isDiscordUser && user.avatar ? (
+              isRegularUser && user.avatar ? (
                 <img src={user.avatar} alt="" className="hub-user-avatar" />
               ) : (
                 <span className="hub-user-icon">{isAdmin ? '\uD83D\uDD27' : '\uD83D\uDC64'}</span>
@@ -435,11 +438,12 @@ export default function App() {
         />
       )}
 
-      {/* User Settings (Discord users) */}
-      {showUserSettings && isDiscordUser && user.discordId && (
+      {/* User Settings (Discord + Steam users) */}
+      {showUserSettings && isRegularUser && (
         <UserSettings
           user={{
-            discordId: user.discordId,
+            id: user.discordId || user.steamId || '',
+            provider: user.provider as 'discord' | 'steam',
             username: user.username ?? '',
             avatar: user.avatar ?? null,
             globalName: user.globalName ?? null,