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/exchange/page.tsx

@@ -3,9 +3,11 @@
 import { useEffect, useState, useCallback, useMemo, ChangeEvent, CSSProperties } from "react";
 import { useRouter } from "next/navigation";
 import { Permission } from "../auth-context";
-import { api, ApiError } from "../api";
+import { api } from "../api";
+import { extractApiErrorMessage } from "../utils/error-handling";
 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 { formatDate, formatTime, getDateRange } from "../utils/date";
@@ -325,18 +327,7 @@ export default function ExchangePage() {
       // Redirect to trades page after successful booking
       router.push("/trades");
     } catch (err) {
-      let errorMessage = "Failed to book trade";
-      if (err instanceof ApiError) {
-        // Extract detail from API error response
-        if (err.data && typeof err.data === "object") {
-          const data = err.data as { detail?: string };
-          errorMessage = data.detail || err.message;
-        } else {
-          errorMessage = err.message;
-        }
-      } else if (err instanceof Error) {
-        errorMessage = err.message;
-      }
+      const errorMessage = extractApiErrorMessage(err, "Failed to book trade");
       setError(errorMessage);
 
       // Check if it's a "same day" error and extract trade public_id (UUID)
@@ -352,11 +343,7 @@ export default function ExchangePage() {
   };
 
   if (isLoading) {
-    return (
-      <main style={layoutStyles.main}>
-        <div style={layoutStyles.loader}>Loading...</div>
-      </main>
-    );
+    return <LoadingState />;
   }
 
   if (!isAuthorized) {