fix: persist image generation results to DB for reload survival, bump v0.17.2

Root cause: after image generation, the message content in DB stayed as
`image-gen-request` (the options form). On app restart, MessageItem re-parsed
this as a generation form instead of showing the completed result. The result
only lived in React state and localStorage.

Fix:
- db.ts: add updateMessageContent() to update a message's content by ID
- /api/chat/messages: add PUT handler for message content updates
- ImageGenConfirmation: accept messageId prop; on successful generation,
  replace message content from image-gen-request to image-gen-result format
  (with localPath references) via PUT API call
- MessageItem: pass message.id to ImageGenConfirmation

Now on restart, parseImageGenResult() matches the persisted image-gen-result
block and renders ImageGenCard directly with the saved image paths.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "codepilot",
-  "version": "0.17.1",
+  "version": "0.17.2",
   "private": true,
   "author": {
     "name": "op7418",