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.

components/ui/spotlight.tsx

@@ -0,0 +1,55 @@
+import { cn } from "@/lib/utils";
+
+type SpotlightProps = {
+  className?: string;
+  fill?: string;
+};
+
+export const Spotlight = ({ className, fill }: SpotlightProps) => {
+  return (
+    <svg
+      className={cn(
+        "animate-spotlight pointer-events-none absolute z-[1] h-[169%] w-[138%] opacity-0 lg:w-[84%]",
+        className
+      )}
+      xmlns="http://www.w3.org/2000/svg"
+      viewBox="0 0 3787 2842"
+      fill="none"
+    >
+      <g filter="url(#filter)">
+        <ellipse
+          cx="1924.71"
+          cy="273.501"
+          rx="1924.71"
+          ry="273.501"
+          transform="matrix(-0.822377 -0.568943 -0.568943 0.822377 3631.88 2291.09)"
+          fill={fill || "white"}
+          fillOpacity="0.21"
+        ></ellipse>
+      </g>
+      <defs>
+        <filter
+          id="filter"
+          x="0.860352"
+          y="0.838989"
+          width="3785.16"
+          height="2840.26"
+          filterUnits="userSpaceOnUse"
+          colorInterpolationFilters="sRGB"
+        >
+          <feFlood floodOpacity="0" result="BackgroundImageFix"></feFlood>
+          <feBlend
+            mode="normal"
+            in="SourceGraphic"
+            in2="BackgroundImageFix"
+            result="shape"
+          ></feBlend>
+          <feGaussianBlur
+            stdDeviation="151"
+            result="effect1_foregroundBlur_1065_8"
+          ></feGaussianBlur>
+        </filter>
+      </defs>
+    </svg>
+  );
+};