feat: add OAuth proxy support to eliminate CORS issues

Add server-side proxy endpoints for OAuth discovery, client registration,
and token exchange to avoid CORS issues when using proxy connection mode.

Server proxy endpoints:
- GET /oauth/resource-metadata - proxies OAuth protected resource metadata
- GET /oauth/metadata - proxies OAuth authorization server metadata
- POST /oauth/register - proxies dynamic client registration
- POST /oauth/token - proxies token exchange

Client changes:
- Construct correct RFC 9728/8414 path-aware well-known URLs client-side
- Route all OAuth discovery through proxy when in proxy connection mode
- Eliminate direct browser fetches that fail with CORS in proxy mode

client/src/App.tsx

@@ -622,9 +622,14 @@ const App = () => {
         };
 
         try {
-          const stateMachine = new OAuthStateMachine(sseUrl, (updates) => {
-            currentState = { ...currentState, ...updates };
-          });
+          const stateMachine = new OAuthStateMachine(
+            sseUrl,
+            (updates) => {
+              currentState = { ...currentState, ...updates };
+            },
+            connectionType,
+            config,
+          );
 
           while (
             currentState.oauthStep !== "complete" &&
@@ -1264,6 +1269,8 @@ const App = () => {
         onBack={() => setIsAuthDebuggerVisible(false)}
         authState={authState}
         updateAuthState={updateAuthState}
+        connectionType={connectionType}
+        config={config}
       />
     </TabsContent>
   );