design: cyan -> rgb(12,96,96) in light theme, #00F0FF stays in dark theme

- globals.css: add --cyan-ai CSS variable (rgb(12,96,96) light / #00F0FF dark)
  + opacity variants --cyan-ai-10/20/30/40/50/80
- Bulk replace all #00F0FF hardcoded Tailwind arbitrary values with
  var(--cyan-ai) / var(--cyan-ai-10) / etc across 55+ TSX/TS files
- Shadow glow colours also updated (suppressed by box-shadow:none in light mode)
- TypeScript: 0 errors

src/components/AI/FeatureIndex.tsx

@@ -1,4 +1,4 @@
-import Link from "next/link"
+import Link from "next/link"
 
 const features = [
   { name: "Mistake Tracking", href: "/dashboard/ai-coach/mistakes" },
@@ -19,9 +19,9 @@ export function FeatureIndex() {
           <Link
             key={f.name}
             href={f.href}
-            className="inline-flex items-center gap-2 px-3.5 py-1.5 rounded-full text-xs font-extrabold bg-[#00F0FF]/10 border border-[#00F0FF]/30 text-[#00F0FF] hover:bg-[#00F0FF] hover:text-black transition-all shadow-[0_0_12px_rgba(0,240,255,0.1)] group"
+            className="inline-flex items-center gap-2 px-3.5 py-1.5 rounded-full text-xs font-extrabold bg-[var(--cyan-ai-10)] border border-[var(--cyan-ai-30)] text-[var(--cyan-ai)] hover:bg-[var(--cyan-ai)] hover:text-black transition-all shadow-[0_0_12px_rgba(12,96,96,0.1)] group"
           >
-            <span className="size-2 rounded-full bg-[#00F0FF] group-hover:bg-black transition-colors" />
+            <span className="size-2 rounded-full bg-[var(--cyan-ai)] group-hover:bg-black transition-colors" />
             <span>{f.name}</span>
             <span className="text-[10px] opacity-75">→</span>
           </Link>