refactor(frontend): improve code quality and maintainability

- Extract API error handling utility (utils/error-handling.ts)
  - Centralize error message extraction logic
  - Add type guards for API errors
  - Replace duplicated error handling across components

- Create reusable Toast component (components/Toast.tsx)
  - Extract toast notification logic from profile page
  - Support auto-dismiss functionality
  - Consistent styling with shared styles

- Extract form validation debouncing hook (hooks/useDebouncedValidation.ts)
  - Reusable debounced validation logic
  - Clean timeout management
  - Used in profile page for form validation

- Consolidate duplicate styles (styles/auth-form.ts)
  - Use shared style tokens instead of duplicating values
  - Reduce code duplication between auth-form and shared styles

- Extract loading state component (components/LoadingState.tsx)
  - Standardize loading UI across pages
  - Replace duplicated loading JSX patterns
  - Used in profile, exchange, and trades pages

- Fix useRequireAuth dependency array
  - Remove unnecessary hasPermission from dependencies
  - Add eslint-disable comment with explanation
  - Improve hook stability and performance

All frontend tests pass. Linting passes.

frontend/app/components/LoadingState.tsx

@@ -0,0 +1,20 @@
+"use client";
+
+import { layoutStyles } from "../styles/shared";
+
+interface LoadingStateProps {
+  /** Custom loading message (default: "Loading...") */
+  message?: string;
+}
+
+/**
+ * Standard loading state component.
+ * Displays a centered loading message with consistent styling.
+ */
+export function LoadingState({ message = "Loading..." }: LoadingStateProps) {
+  return (
+    <main style={layoutStyles.main}>
+      <div style={layoutStyles.loader}>{message}</div>
+    </main>
+  );
+}