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/trades/page.tsx

@@ -6,6 +6,7 @@ import { Permission } from "../auth-context";
 import { api } from "../api";
 import { Header } from "../components/Header";
 import { SatsDisplay } from "../components/SatsDisplay";
+import { LoadingState } from "../components/LoadingState";
 import { useRequireAuth } from "../hooks/useRequireAuth";
 import { components } from "../generated/api";
 import { formatDateTime } from "../utils/date";
@@ -68,11 +69,7 @@ export default function TradesPage() {
   };
 
   if (isLoading) {
-    return (
-      <main style={layoutStyles.main}>
-        <div style={layoutStyles.loader}>Loading...</div>
-      </main>
-    );
+    return <LoadingState />;
   }
 
   if (!isAuthorized) {