feat(gateway): enhance gateway process management with auto-reconnection

Improve Gateway lifecycle management with the following features:

- Add exponential backoff reconnection (1s-30s delay, max 10 attempts)
- Add health check monitoring every 30 seconds
- Add proper restart method with graceful shutdown
- Handle server-initiated notifications (channel status, chat messages)
- Add 'reconnecting' state for better UI feedback
- Enhance IPC handlers with isConnected and health check endpoints
- Update preload script with new event channels
- Improve type safety and error handling throughout

Also fixes several TypeScript errors and unused variable warnings.

electron/preload/index.ts

@@ -16,10 +16,12 @@ const electronAPI = {
       const validChannels = [
         // Gateway
         'gateway:status',
+        'gateway:isConnected',
         'gateway:start',
         'gateway:stop',
         'gateway:restart',
         'gateway:rpc',
+        'gateway:health',
         // Shell
         'shell:openExternal',
         'shell:showItemInFolder',
@@ -74,6 +76,9 @@ const electronAPI = {
       const validChannels = [
         'gateway:status-changed',
         'gateway:message',
+        'gateway:notification',
+        'gateway:channel-status',
+        'gateway:chat-message',
         'gateway:exit',
         'gateway:error',
         'navigate',
@@ -106,6 +111,9 @@ const electronAPI = {
       const validChannels = [
         'gateway:status-changed',
         'gateway:message',
+        'gateway:notification',
+        'gateway:channel-status',
+        'gateway:chat-message',
         'gateway:exit',
         'gateway:error',
         'navigate',