Implement compact, undo, and init commands

- Add compact command with proper API parameters (providerID, modelID)
- Implement undo command matching TUI behavior:
  - Find previous user message before revert point
  - Restore reverted message to prompt input
  - Filter messages client-side based on session.revert field
- Add init command with proper hex-encoded message ID generation
- Add session.revert field handling:
  - Parse revert field from API responses in fetchSessions and createSession
  - Update revert field via SSE session.updated events
  - Filter messages during render when revert point is set
- Add SSE event handlers for session.compacted and session.error
- Add force reload option to loadMessages for post-compact refresh
- Messages now filter instantly based on revert state without API reload

src/types/session.ts

@@ -14,6 +14,12 @@ export interface Session {
     created: number
     updated: number
   }
+  revert?: {
+    messageID: string
+    partID?: string
+    snapshot?: string
+    diff?: string
+  }
   messages: Message[]
   messagesInfo: Map<string, any>
 }