ui: Motion-driven animation layer (motion.dev)

Unify scroll reveals, hero entrance, nav active-underline, button springs,
the scroll-progress bar and backdrop parallax under one Motion system
(src/scripts/anim.ts), bundled into a self-hosted /_astro module so CSP
script-src 'self' holds. Progress bar + parallax now scroll()-driven and
cross-browser (was Chromium-only animation-timeline). Fail-open, reduced-motion
aware, content fully visible with JS off.

package.json

@@ -19,6 +19,7 @@
     "@astrojs/sitemap": "^3.7.3",
     "@tailwindcss/vite": "^4.3.1",
     "astro": "^7.0.3",
+    "motion": "^12.42.0",
     "tailwindcss": "^4.3.1"
   },
   "devDependencies": {