Feature: Incognito/ephemeral chat mode

[Roasbeef]

Summary

Add support for "incognito" or ephemeral chats that don't persist to conversation history.

Use Cases

• Quick one-off questions without cluttering history
• Sensitive queries users don't want saved
• Testing/experimenting without filling up storage
• Privacy-conscious usage

Proposed Behavior

• New option in header menu: "New Incognito Chat" or toggle
• Visual indicator when in incognito mode (e.g., icon, different header color)
• Messages work normally during the session
• On close or new conversation: no history saved, no entry in history modal
• Session ID still works for multi-turn within the session, just not persisted

UI/UX Considerations

• Clear visual distinction from regular chats
• Warning/confirmation when closing incognito chat (data will be lost)
• Keyboard shortcut for quick access (e.g., Cmd+Shift+N)
• Option in settings to set incognito as default?

Technical Notes

• Skip ConversationManager.addMessage() calls when incognito
• Don't create conversation JSON files
• Could use a flag on ChatView: isIncognito: boolean
• Still allow copy/export of current conversation if needed

Acceptance Criteria

• "New Incognito Chat" option in header dropdown
• Visual indicator for incognito mode
• No history persisted for incognito chats
• Conversation works normally within session
• No entry appears in history modal
• Warning before closing incognito chat with content