fix: dark mode theme switching and styling

- Apply theme immediately when saving settings (fixes theme not updating)
- Add dark mode CSS variables for success/warning/danger/info colors
- Fix shell-header background for dark mode
- Fix URL validation styles to use CSS variables
- Add explicit background/color to form inputs for dark mode support
- Quality gates: typecheck OK, lint OK, build OK

apps/web/src/pages/settings.tsx

@@ -53,6 +53,15 @@ export const SettingsPage = () => {
       const updated = await updateUserConfig(update);
       setConfig(updated);
       setSaveStatus("saved");
+      
+      // Apply theme immediately
+      const theme = updated.theme ?? "system";
+      if (theme === "system") {
+        document.documentElement.removeAttribute("data-theme");
+      } else {
+        document.documentElement.setAttribute("data-theme", theme);
+      }
+      
       setTimeout(() => setSaveStatus("idle"), 2000);
     } catch {
       setSaveStatus("error");