Fix Chromium scrollbar styling to match dark theme

Add custom scrollbar styles for Chromium/Webkit browsers to match the
dark theme aesthetic. Firefox already had proper scrollbar styling, but
Chrome was showing default light gray scrollbars.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

static/css/app.css

@@ -91,6 +91,36 @@
     --shadow-md: 0 4px 6px rgba(0, 0, 0, 0.3);
 }
 
+/* ============================================
+   Scrollbar Styling
+   ============================================ */
+
+/* Firefox scrollbar styling */
+* {
+    scrollbar-width: thin;
+    scrollbar-color: var(--border) var(--surface);
+}
+
+/* Chromium/Webkit scrollbar styling */
+*::-webkit-scrollbar {
+    width: 12px;
+    height: 12px;
+}
+
+*::-webkit-scrollbar-track {
+    background: var(--surface);
+}
+
+*::-webkit-scrollbar-thumb {
+    background-color: var(--border);
+    border-radius: 6px;
+    border: 3px solid var(--surface);
+}
+
+*::-webkit-scrollbar-thumb:hover {
+    background-color: var(--surface-hover);
+}
+
 /* ============================================
    Reset & Base
    ============================================ */