fix: trigger browser password save prompt on password change

Safari, Chrome, and Firefox only offer to save passwords when they see
a real form submission with page navigation, not fetch() + redirect.

After the change-password API call succeeds, dynamically create a form
with the username and new password (autocomplete=username + new-password),
POST it to "/" causing a real navigation. The browser detects the form
submission with credential fields and prompts to save.

Also make the username field visible (read-only) on the change-password
page since Safari ignores hidden inputs for password detection, and add
autocomplete attributes to the login page fields.

apps/web/src/pages/login-page.tsx

@@ -57,6 +57,8 @@ export function LoginPage() {
               <input
                 id="username"
                 type="text"
+                name="username"
+                autoComplete="username"
                 value={username}
                 onChange={(e) => setUsername(e.target.value)}
                 placeholder="Enter username"
@@ -71,6 +73,8 @@ export function LoginPage() {
               <input
                 id="password"
                 type="password"
+                name="password"
+                autoComplete="current-password"
                 value={password}
                 onChange={(e) => setPassword(e.target.value)}
                 placeholder="Enter your password"