handleContentChange now syncs content to tab state on every editor
change (not just on Cmd+S save). This ensures the AI panel's context
snapshot always contains the current editor body, fixing the bug where
the AI reported empty note bodies for unsaved edits.
Root cause: pendingContentRef buffered content for save but never
updated notes.tabs[].content, so activeTab?.content (used by the AI
context builder) always reflected the last-saved disk content.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>