fix: replace all hardcoded hex colors with CSS theme variables

Phase 15c — theme-consistent color system:

Replaced 63 instances of hardcoded hex colors (#5e6ad2, #7170ff, #4f46e5,
#6a75e0, #828fff, #94a3b8, etc.) with CSS variables from globals.css:
- #5e6ad2 → var(--brand) (light: #5e6ad2, dark: #5e6ad2)
- #7170ff → var(--accent) (light: #4f46e5, dark: #7170ff)
- #4f46e5 → var(--accent)
- #6a75e0 → var(--brand-hover) (new var, light: #6a75e0, dark: #7170ff)
- #828fff → var(--accent-hover)
- #868686 → var(--text-dim)
- Dark-mode-specific hex in Markdown.tsx → CSS variables

Added --brand-hover CSS variable (light: #6a75e0, dark: #7170ff)
Added dark:prose-invert instead of prose-invert (light mode now uses
default prose theme instead of forced dark)

Files changed: 15 component files + globals.css

apps/web/app/layout.tsx

@@ -35,7 +35,7 @@ export default function RootLayout({
           }}
         />
       </head>
-      <body className="min-h-screen bg-[var(--bg-app)] text-[var(--text-secondary)] font-sans antialiased selection:bg-[#5e6ad2]/30 selection:text-white flex flex-col transition-colors duration-150">
+      <body className="min-h-screen bg-[var(--bg-app)] text-[var(--text-secondary)] font-sans antialiased selection:bg-[var(--brand)]/30 selection:text-white flex flex-col transition-colors duration-150">
         <div hidden>
           <div id="a11y-live-region" aria-live="polite" aria-atomic="true" />
         </div>