Animate gradient background spotlights via canvas

[Copilot]

The static CSS radial-gradient background had no movement. Replace it with a canvas-driven animation so the three spotlight orbs drift and pulse continuously.

Changes

• _layouts/default.html — swap <div class="page-field"> for <canvas id="page-canvas" aria-hidden="true">
• assets/css/styles.scss — replace .page-field background rule with positional styles for #page-canvas (fixed, full-bleed, z-index: -2)
• assets/js/site.js — canvas animation loop:
  • Redraws the navy base + three orbs (blue upper-left, lime upper-right, deep-navy lower-right) every frame via createRadialGradient
  • Each orb drifts slowly around its origin (bounces at ±12% viewport offset) and pulses in size (±18% of base radius)
  • Orbs are drawn as ellipses via ctx.scale to preserve the original asymmetric radii
  • Pauses via Page Visibility API when tab is hidden; respects prefers-reduced-motion (renders static if set)