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/hooks/use-theme.ts

@@ -0,0 +1,23 @@
+import { useEffect } from "react";
+
+import { getUserConfig } from "../api/settings";
+
+export function useTheme() {
+  useEffect(() => {
+    void (async () => {
+      try {
+        const config = await getUserConfig();
+        const theme = config.theme ?? "system";
+        
+        if (theme === "system") {
+          // Remove any explicit theme class and let system decide
+          document.documentElement.removeAttribute("data-theme");
+        } else {
+          document.documentElement.setAttribute("data-theme", theme);
+        }
+      } catch {
+        // Silently fail - theme is not critical
+      }
+    })();
+  }, []);
+}