Feat: Implement real GitHub Copilot Device Flow authentication UI

- Add IPC handlers for github-copilot:authenticate, isAuthenticated, disconnect
- Add schema definitions for new IPC handlers in shared/ipc.ts
- Implement Device Flow authentication button with real flow
- Show device code and verification URL to user when authenticating
- Automatically open GitHub verification page in browser
- Auto-load models after successful authentication
- Update GitHub Copilot model list (gpt-4o, gpt-4-turbo, gpt-3.5-turbo, claude-3.5-sonnet)
- Display loading state and error messages in UI
- Properly integrate with existing auth service

This fixes the broken authentication flow where the button didn't do anything.

apps/x/packages/shared/src/ipc.ts

@@ -257,6 +257,39 @@ const ipcSchemas = {
       })),
     }),
   },
+  'github-copilot:authenticate': {
+    req: z.null(),
+    res: z.union([
+      z.object({
+        success: z.literal(true),
+        userCode: z.string(),
+        verificationUri: z.string(),
+        expiresIn: z.number(),
+      }),
+      z.object({
+        success: z.literal(false),
+        error: z.string(),
+      }),
+    ]),
+  },
+  'github-copilot:isAuthenticated': {
+    req: z.null(),
+    res: z.object({
+      authenticated: z.boolean(),
+    }),
+  },
+  'github-copilot:disconnect': {
+    req: z.null(),
+    res: z.union([
+      z.object({
+        success: z.literal(true),
+      }),
+      z.object({
+        success: z.literal(false),
+        error: z.string(),
+      }),
+    ]),
+  },
   'account:getRowboat': {
     req: z.null(),
     res: z.object({