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/Sidebar.tsx

@@ -106,7 +106,7 @@ function TreeNodeItem({
       <div
         className={`group flex items-center justify-between gap-1.5 text-xs py-1.5 px-2 rounded-md transition-all ${
           isActive
-            ? "text-[#5e6ad2] dark:text-[#7170ff] bg-[#5e6ad2]/15 font-semibold border border-[#5e6ad2]/30 shadow-sm"
+            ? "text-[var(--brand)] dark:text-[var(--accent)] bg-[var(--brand)]/15 font-semibold border border-[var(--brand)]/30 shadow-sm"
             : isParentOfActive
               ? "text-[var(--text-primary)] bg-[var(--bg-elevated)]"
               : "text-[var(--text-muted)] hover:text-[var(--text-primary)] hover:bg-[var(--bg-elevated)]"
@@ -263,7 +263,7 @@ export default function Sidebar() {
                 href={`/${id}`}
                 className={`flex items-center justify-between gap-1.5 px-1 py-1 mb-1 rounded text-xs font-semibold uppercase tracking-wider transition-colors ${
                   isSectionActive
-                    ? "text-[#5e6ad2] dark:text-[#7170ff]"
+                    ? "text-[var(--brand)] dark:text-[var(--accent)]"
                     : "text-[var(--text-muted)] hover:text-[var(--text-primary)]"
                 }`}
               >