feat: AI chat UX

[datlechin]

Summary

5 UX improvements to make the AI chat feel like a proper native macOS citizen:

• U1 — User message tinted bubbles: User messages now have a subtle accent-colored background (Color.accentColor.opacity(0.06)) with rounded corners, while assistant messages remain on the default background. Makes scanning conversations much easier.

• U12 — Edit and resend: User messages show a pencil edit button. Clicking it populates the input field with the message content and removes the message + all subsequent messages, letting the user modify and re-send. Standard in all AI chat UIs.

• U5 — Scroll-to-bottom button: A floating arrow.down.circle.fill button appears when the user scrolls up during streaming. Clicking it jumps back to the latest message. Uses ScrollViewProxy stored in state for access outside the ScrollViewReader scope.

• U6 — Connection name in header: The chat header now shows the database connection name as a subtitle below the conversation title. Users can immediately see which database the AI is talking about.

• U11 — Insert button feedback: The "Insert" button on code blocks is now properly disabled when no editor is focused, with a tooltip explaining "Focus the query editor to insert". Previously it silently did nothing.

Test plan

• Build succeeds
• User messages display with tinted background, assistant messages plain
• Click edit (pencil) on user message → input field populated, message removed
• Scroll up during streaming → floating arrow button appears
• Click floating button → scrolls to bottom
• Connection name visible below conversation title in header
• Insert button disabled with tooltip when editor not focused
• Insert button works normally when editor is focused