feat: add note to myself mode

[jeba57]

Summary

• Add a "Note to myself" option at the bottom of the AI model dropdown
• Allow users to write notes without sending a request to an AI provider
• Keep notes out of the AI conversation context
• Persist notes in chat history, reusing existing Dexie helpers (no new storage system)
• Restore notes when reopening a saved chat
• Respect existing Temporary Chat setting (notes aren't persisted when enabled)
• Keep normal AI chat behavior unchanged
• Scope: Side Panel only

Testing

• Created a note and confirmed no request was sent to Ollama (checked network tab)
• Confirmed the note appears in the conversation with a distinct visual tag
• Reopened the chat from history and confirmed the note persisted
• Sent a normal message afterward and confirmed the note wasn't included in the AI's context
• Switched back to a real model and confirmed normal Ollama responses still work
• No new TypeScript errors introduced

Screenshots

Fixes #746