feat(ui): reverse sheet primary button colors, fix liquid QR icon

[Kelbie]

What / Why

• Sheet primary buttons (e.g. "Generate new account") now use reversed colors: foreground as background, background as text — higher contrast and clearer CTA
• Liquid glass QR button icon now uses foreground/white instead of dimmed shade-300 — better visibility on glass

What changed

• PopupHost: added getSheetButtonClassName/getSheetButtonLabelClassName, applied to all sheet primary buttons (standard, custom footer, profile switcher)
• QRButton.ios: pass color (foreground) to liquid variant instead of shadeColor300
• Prettier: fix tabs layout trailing newline

Test plan

1. Open any sheet with primary button (profile switcher, camera permission, etc.) — primary should show light text on dark bg
2. On liquid-glass device, QR button icon should be white/foreground
3. npm run lint && npm run type-check && npm run pretty:check && npm run knip && npm test — all pass

Notes / Risks

• None

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