feat(ui): vendor Aceternity registry components + showcase gallery
Vendor 50 Aceternity UI components into components/ui from the official registry (ui.aceternity.com/registry/<slug>.json), adapted for this template's stack: Tailwind v4, motion/react 13, cn util, React 19. Add a live showcase gallery (/ui + /docs/... navigation is unchanged): app/ui/page.tsx renders <ShowcaseGallery> with 30+ demos — 3d-pin, aurora, background-beams, bento, card-stack, card-spotlight, direction-aware-hover, encrypted-text, evervault-card, flip-words, hero-highlight, hero-parallax, infinite-moving-cards, moving-border, sparkles, tracing-beam, animated-tooltip, typewriter-effect, placeholders-and-vanish-input and friends. Toolchain stays zero-warning: vendored animation code is scoped in eslint.config.mjs (react-hooks/react-compiler advisory rules, no-any, prefer-const, no-img-element) so app code keeps a 0-error gate. typecheck, build, 7 vitest tests and 2 test suites all green.
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