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/index.ts

@@ -0,0 +1,26 @@
+/**
+ * IPC handlers aggregator
+ *
+ * Registers all IPC handlers in one place.
+ */
+
+import { registerDialogHandlers } from './dialog-handlers';
+import { registerShellHandlers } from './shell-handlers';
+import { registerAppHandlers } from './app-handlers';
+import { registerAuthHandlers } from './auth-handlers';
+import { registerWindowHandlers } from './window-handlers';
+import { registerServerHandlers } from './server-handlers';
+
+export { IPC_CHANNELS } from './channels';
+
+/**
+ * Register all IPC handlers
+ */
+export function registerAllHandlers(): void {
+  registerDialogHandlers();
+  registerShellHandlers();
+  registerAppHandlers();
+  registerAuthHandlers();
+  registerWindowHandlers();
+  registerServerHandlers();
+}