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/components/McpConfigSnippet.tsx

@@ -70,7 +70,7 @@ export default function McpConfigSnippet() {
               onClick={() => setActiveTab(c.id)}
               className={`px-3 py-1 text-xs rounded-md font-medium transition-all ${
                 activeTab === c.id
-                  ? "bg-[#5e6ad2] text-white shadow-sm"
+                  ? "bg-[var(--brand)] text-white shadow-sm"
                   : "text-[var(--text-muted)] hover:text-[var(--text-primary)] hover:bg-[var(--bg-surface)]"
               }`}
             >