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/utils/port-manager.ts

@@ -0,0 +1,42 @@
+/**
+ * Port management utilities
+ *
+ * Functions for checking port availability and finding open ports.
+ * No Electron dependencies - pure utility module.
+ */
+
+import net from 'net';
+
+/**
+ * Check if a port is available
+ */
+export function isPortAvailable(port: number): Promise<boolean> {
+  return new Promise((resolve) => {
+    const server = net.createServer();
+    server.once('error', () => {
+      resolve(false);
+    });
+    server.once('listening', () => {
+      server.close(() => {
+        resolve(true);
+      });
+    });
+    // Use Node's default binding semantics (matches most dev servers)
+    // This avoids false-positives when a port is taken on IPv6/dual-stack.
+    server.listen(port);
+  });
+}
+
+/**
+ * Find an available port starting from the preferred port
+ * Tries up to 100 ports in sequence
+ */
+export async function findAvailablePort(preferredPort: number): Promise<number> {
+  for (let offset = 0; offset < 100; offset++) {
+    const port = preferredPort + offset;
+    if (await isPortAvailable(port)) {
+      return port;
+    }
+  }
+  throw new Error(`Could not find an available port starting from ${preferredPort}`);
+}