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/create/page.tsx

@@ -34,7 +34,7 @@ export default async function CreatePage() {
       {/* Header */}
       <div className="flex items-center justify-between pb-4 border-b border-[var(--border-color)]">
         <div className="flex items-center gap-3">
-          <div className="w-10 h-10 rounded-lg bg-[#5e6ad2]/15 border border-[#5e6ad2]/30 flex items-center justify-center text-xl text-[#5e6ad2] dark:text-[#7170ff]">
+          <div className="w-10 h-10 rounded-lg bg-[var(--brand)]/15 border border-[var(--brand)]/30 flex items-center justify-center text-xl text-[var(--brand)] dark:text-[var(--accent)]">
             ✏️
           </div>
           <div>