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/channels.ts

@@ -0,0 +1,36 @@
+/**
+ * IPC channel constants
+ *
+ * Single source of truth for all IPC channel names.
+ * Used by both main process handlers and preload script.
+ */
+
+export const IPC_CHANNELS = {
+  DIALOG: {
+    OPEN_DIRECTORY: 'dialog:openDirectory',
+    OPEN_FILE: 'dialog:openFile',
+    SAVE_FILE: 'dialog:saveFile',
+  },
+  SHELL: {
+    OPEN_EXTERNAL: 'shell:openExternal',
+    OPEN_PATH: 'shell:openPath',
+    OPEN_IN_EDITOR: 'shell:openInEditor',
+  },
+  APP: {
+    GET_PATH: 'app:getPath',
+    GET_VERSION: 'app:getVersion',
+    IS_PACKAGED: 'app:isPackaged',
+    QUIT: 'app:quit',
+  },
+  AUTH: {
+    GET_API_KEY: 'auth:getApiKey',
+    IS_EXTERNAL_SERVER_MODE: 'auth:isExternalServerMode',
+  },
+  WINDOW: {
+    UPDATE_MIN_WIDTH: 'window:updateMinWidth',
+  },
+  SERVER: {
+    GET_URL: 'server:getUrl',
+  },
+  PING: 'ping',
+} as const;