Security, code quality and frontend improvements

- Move SECRET_KEY out of docker-compose into .env (env_file), add .env.example
- Add flask-limiter with 10 req/min on login route; introduce util/limiter.py
- Replace direct mysql.connector.connect() calls with MySQLConnectionPool via util/db_pool.py
- Fix deprecated datetime.utcnow() -> datetime.now(timezone.utc) in auth/login.py
- Remove dead /api/scripts 410 route from admin.py
- Add MD5 security warning in Md5Tool.jsx
- Add ErrorBoundary component and wrap App.jsx
- Expand README with setup guide, screenshot and project structure

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/App.jsx

@@ -6,6 +6,7 @@ import Md5Tool from './components/Md5Tool';
 import NavBar from './components/NavBar';
 import ToolOverview from './components/ToolOverview';
 import AdminDashboard from './components/AdminDashboard';
+import ErrorBoundary from './components/ErrorBoundary';
 
 
 import './css/base.css';
@@ -21,23 +22,25 @@ function App() {
   const role = localStorage.getItem('role');
 
   return (
-    <BrowserRouter>
-      <NavBar />
-        <Routes>
-          <Route path="/" element={isLoggedIn ? <ToolOverview /> : <Navigate to="/login" />} />
-          <Route path="/login" element={<LoginForm />} />
-          {/*<Route path="/register" element={<RegisterForm />} />*/}
-          <Route path="/tools/md5" element={isLoggedIn ? <Md5Tool /> : <Navigate to="/login" />} />
-          <Route
-            path="/admin"
-            element={
-              isLoggedIn && role === 'admin'
-                ? <AdminDashboard />
-                : <Navigate to="/" />
-            }
-          />
-        </Routes>
-    </BrowserRouter>
+    <ErrorBoundary>
+      <BrowserRouter>
+        <NavBar />
+          <Routes>
+            <Route path="/" element={isLoggedIn ? <ToolOverview /> : <Navigate to="/login" />} />
+            <Route path="/login" element={<LoginForm />} />
+            {/*<Route path="/register" element={<RegisterForm />} />*/}
+            <Route path="/tools/md5" element={isLoggedIn ? <Md5Tool /> : <Navigate to="/login" />} />
+            <Route
+              path="/admin"
+              element={
+                isLoggedIn && role === 'admin'
+                  ? <AdminDashboard />
+                  : <Navigate to="/" />
+              }
+            />
+          </Routes>
+      </BrowserRouter>
+    </ErrorBoundary>
   );
 }