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/styles.css

@@ -2456,9 +2456,9 @@ html, body {
   border-color: #5865F2;
   background: rgba(88, 101, 242, 0.08);
 }
-.hub-login-provider-btn.steam:hover:not(:disabled) {
-  border-color: #1b2838;
-  background: rgba(27, 40, 56, 0.15);
+.hub-login-provider-btn.steam:hover {
+  border-color: #66c0f4;
+  background: rgba(102, 192, 244, 0.08);
 }
 .hub-login-provider-btn.admin:hover {
   border-color: var(--accent);