## What
Two mobile-header fixes:
- **Logo no longer wraps/clips.** The brand row had no `shrink-0`, so
flexbox squeezed it and "Whisper Money" wrapped to two lines or got cut
off by the buttons. Now pinned with `shrink-0` + `whitespace-nowrap`,
and the bird icon is bumped `size-4` → `size-5`.
- **Auth buttons fit when logged out.** Log in + Register both as text
buttons overflowed the narrow pill. Log in is now an icon-only ghost
button (`LogIn` icon, `aria-label` for a11y), so Register keeps its full
primary-CTA pill.
Desktop header is untouched.
## Test plan
- [ ] Logged-out mobile: logo on one line, login icon + Register both
visible, no overflow
- [ ] Logged-in mobile: Dashboard button unaffected
- [ ] Desktop unchanged
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