enhance error handling for register

public/s/js/f0ck.js

@@ -126,6 +126,58 @@ window.requestAnimFrame = (function () {
         registerModal.style.display = 'none';
       }
     });
+
+    const registerForm = registerModal.querySelector('form');
+    if (registerForm) {
+      registerForm.addEventListener('submit', async (e) => {
+        e.preventDefault();
+        const formData = new FormData(registerForm);
+        const params = new URLSearchParams(formData);
+
+        try {
+          const res = await fetch('/register', {
+            method: 'POST',
+            headers: {
+              'X-Requested-With': 'XMLHttpRequest',
+              'Accept': 'application/json',
+              'Content-Type': 'application/x-www-form-urlencoded'
+            },
+            body: params
+          });
+
+          if (res.redirected) {
+            window.location.href = res.url;
+            return;
+          }
+
+          const text = await res.text();
+          let json;
+          try {
+            json = JSON.parse(text);
+          } catch (e) { }
+
+          if (json && json.success === false) {
+            let errDiv = registerForm.querySelector('.flash-error');
+            if (!errDiv) {
+              errDiv = document.createElement('div');
+              errDiv.className = 'flash-error';
+              errDiv.style.color = '#ff4444';
+              errDiv.style.textAlign = 'center';
+              errDiv.style.marginBottom = '10px';
+              errDiv.style.fontWeight = 'bold';
+              registerForm.insertBefore(errDiv, registerForm.firstChild); // Insert before h2 or inputs
+              // Actually firstChild is text or h2. Let's insert after H2 if possible?
+              // The form has H2 as first element roughly.
+              // insertBefore firstChild is fine, it puts it at top.
+            }
+            errDiv.textContent = json.msg;
+          }
+
+        } catch (err) {
+          console.error('Registration error:', err);
+        }
+      });
+    }
   }
 
   // Initialize background preference

src/inc/routes/register.mjs

@@ -15,6 +15,9 @@ export default (router, tpl) => {
         const { username, password, password_confirm, token } = req.post;
 
         const renderError = (msg) => {
+            if (req.headers['x-requested-with'] === 'XMLHttpRequest' || (req.headers.accept && req.headers.accept.includes('application/json'))) {
+                return res.writeHead(200, { 'Content-Type': 'application/json' }).end(JSON.stringify({ success: false, msg }));
+            }
             return res.reply({
                 body: tpl.render("register", { theme: req.cookies.theme ?? "f0ck", error: msg })
             });