Redesign NavBar #62

Bring NavBar to shadcn-registry quality. Depends on #58.

• Blurred translucent bar; new variant="floating" (inset rounded card).
• Spring hover highlight across links; activeHref sets aria-current="page".
• shadcn primary CTA, ghost GitHub button, language menu matching LanguageSwitcher, globe fallback when showFlags is off.
• Animated mobile menu with morphing hamburger; CTA and GitHub link in the mobile panel.
• classNames slots. Backward compatible.