feat: complete user config management

- Add theme support with dark/light/system modes
- Add useTheme hook for applying user config theme
- Update router to use SettingsPage
- Update app-shell to apply theme on load
- Add CSS variables for dark theme
- Fix mypy errors in user_config.py
- Quality gates pass: ruff, mypy, typecheck, lint, build

apps/web/src/styles.css

@@ -10,16 +10,31 @@
   --border: #d8d0c5;
 }
 
+[data-theme="dark"] {
+  color-scheme: dark;
+  --bg: #1a1a18;
+  --panel: #252522;
+  --ink: #e8e6e1;
+  --muted: #a39e96;
+  --brand: #4a9e7f;
+  --brand-strong: #3d8a6e;
+  --border: #3d3d38;
+}
+
 * {
   box-sizing: border-box;
 }
 
 body {
   margin: 0;
-  background: radial-gradient(circle at top right, #fff5d6, var(--bg));
+  background: var(--bg);
   color: var(--ink);
 }
 
+[data-theme="dark"] body {
+  background: radial-gradient(circle at top right, #2a2520, var(--bg));
+}
+
 a {
   color: inherit;
   text-decoration: none;