feat(ui): electric Bat-Computer redesign — cyan→magenta theme

Cinematic full-page backdrop: per-theme Bat-Computer board image,
scroll-panned (S→E→SE→NE) via CSS scroll-driven animation behind a
legibility scrim — replaces the grid/aura motif.

- Accent retuned royal-blue → the board's cyan→magenta; headline runs a
  cyan→magenta electric sweep; gradient primary CTA + scroll-progress line.
- Display face: self-hosted Chakra Petch (OFL) on hero + section titles.
- Skill chips reworked as circuit-node chips (glowing via + gradient hover
  outline); frosted-glass cards with a cyan→magenta hover hairline; section
  eyebrows get a gradient tick.
- Remove unused BackgroundGrid component + grid assets.

All CSS-only / zero external requests / no inline JS — strict CSP and A+
security headers preserved.

src/components/Section.astro

@@ -25,11 +25,27 @@ const { id, eyebrow, title, index } = Astro.props;
   .section__head {
     margin-bottom: var(--space-6);
   }
+  /* A short cyan→magenta tick leads each section eyebrow — board palette. */
+  .section__head .eyebrow {
+    display: inline-flex;
+    align-items: center;
+    gap: 0.6rem;
+  }
+  .section__head .eyebrow::before {
+    content: "";
+    width: 1.7rem;
+    height: 2px;
+    border-radius: 2px;
+    background: linear-gradient(90deg, var(--accent), var(--accent-2));
+    box-shadow: 0 0 8px var(--accent-glow);
+  }
   .section__index {
     color: var(--text-faint);
     margin-right: 0.6rem;
   }
   .section__title {
+    font-family: var(--font-display);
+    letter-spacing: -0.01em;
     margin-top: var(--space-3);
     font-size: var(--step-3);
     max-width: 22ch;