Add tips/hints on empty chat screen for feature discovery

## Context
Help users discover features like `/note:` command.

## Design
Show rotating tips on empty chat home screen:
- "Tip: Type /note: to save a quick note to your space"
- "Tip: Right-click sources to add papers to your space"
- etc.

## Acceptance Criteria
- [ ] Tips shown on empty FloatingInputView screen
- [ ] Tips rotate or show randomly
- [ ] Non-intrusive, subtle styling