refactor: Modularize Electron main process into single-responsibility components

Extract the monolithic main.ts (~1000 lines) into focused modules:

- electron/constants.ts - Window sizing, port defaults, filenames
- electron/state.ts - Shared state container
- electron/utils/ - Port availability and icon utilities
- electron/security/ - API key management
- electron/windows/ - Window bounds and main window creation
- electron/server/ - Backend and static server management
- electron/ipc/ - IPC handlers with shared channel constants

Benefits:
- Improved testability with isolated modules
- Better discoverability and maintainability
- Single source of truth for IPC channels (used by both main and preload)
- Clear separation of concerns

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/ui/src/electron/ipc/auth-handlers.ts

@@ -0,0 +1,29 @@
+/**
+ * Auth IPC handlers
+ *
+ * Handles authentication-related operations.
+ */
+
+import { ipcMain } from 'electron';
+import { IPC_CHANNELS } from './channels';
+import { state } from '../state';
+
+/**
+ * Register auth IPC handlers
+ */
+export function registerAuthHandlers(): void {
+  // Get API key for authentication
+  // Returns null in external server mode to trigger session-based auth
+  ipcMain.handle(IPC_CHANNELS.AUTH.GET_API_KEY, () => {
+    if (state.isExternalServerMode) {
+      return null;
+    }
+    return state.apiKey;
+  });
+
+  // Check if running in external server mode (Docker API)
+  // Used by renderer to determine auth flow
+  ipcMain.handle(IPC_CHANNELS.AUTH.IS_EXTERNAL_SERVER_MODE, () => {
+    return state.isExternalServerMode;
+  });
+}