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/auth-context.tsx

@@ -2,8 +2,9 @@
 
 import { createContext, useContext, useState, useEffect, useCallback, ReactNode } from "react";
 
-import { api, ApiError } from "./api";
+import { api } from "./api";
 import { components } from "./generated/api";
+import { extractApiErrorMessage } from "./utils/error-handling";
 
 // Permission type from generated OpenAPI schema
 export type PermissionType = components["schemas"]["Permission"];
@@ -67,11 +68,7 @@ export function AuthProvider({ children }: { children: ReactNode }) {
       const userData = await api.post<User>("/api/auth/login", { email, password });
       setUser(userData);
     } catch (err) {
-      if (err instanceof ApiError) {
-        const data = err.data as { detail?: string };
-        throw new Error(data?.detail || "Login failed");
-      }
-      throw err;
+      throw new Error(extractApiErrorMessage(err, "Login failed"));
     }
   };
 
@@ -84,11 +81,7 @@ export function AuthProvider({ children }: { children: ReactNode }) {
       });
       setUser(userData);
     } catch (err) {
-      if (err instanceof ApiError) {
-        const data = err.data as { detail?: string };
-        throw new Error(data?.detail || "Registration failed");
-      }
-      throw err;
+      throw new Error(extractApiErrorMessage(err, "Registration failed"));
     }
   };