refactor(frontend): modernize login page with AntD primitives

- Theme cycle button switched from `<button.theme-cycle>` + custom CSS
  to AntD `<Button shape="circle" className="toolbar-btn">` (matches
  sub page chrome already established).
- Theme icons switched from hand-rolled inline SVG (sun, moon,
  moon+star) to AntD `<SunOutlined />`, `<MoonOutlined />`,
  `<MoonFilled />` for the three light / dark / ultra-dark states.
- Language popover content switched from `<ul.lang-list>` +
  `<button.lang-item>` to AntD `<Menu mode="vertical" selectable />`
  with `selectedKeys=[lang]`; native hover / keyboard nav / active
  highlight come for free.
- Drop CSS for `.theme-cycle`, `.lang-list`, `.lang-item*` (now unused).
  `.toolbar-btn` retained since it sizes both circular buttons.

frontend/src/pages/login/LoginPage.css

@@ -228,36 +228,6 @@
   font-size: 18px;
 }
 
-.theme-cycle {
-  width: 40px;
-  height: 40px;
-  border-radius: 50%;
-  border: 1px solid var(--color-border);
-  background: var(--bg-card);
-  color: var(--color-text);
-  display: inline-flex;
-  align-items: center;
-  justify-content: center;
-  cursor: pointer;
-  padding: 0;
-  -webkit-backdrop-filter: blur(20px);
-  backdrop-filter: blur(20px);
-  transition: background-color 0.2s, transform 0.15s, color 0.2s;
-}
-
-.theme-cycle:hover,
-.theme-cycle:focus-visible {
-  background-color: rgba(99, 102, 241, 0.15);
-  color: var(--color-accent);
-  transform: scale(1.05);
-  outline: none;
-}
-
-.theme-cycle svg {
-  width: 18px;
-  height: 18px;
-}
-
 .login-wrapper {
   position: relative;
   min-height: 100vh;
@@ -402,44 +372,3 @@
   margin-bottom: 0;
 }
 
-.lang-list {
-  list-style: none;
-  margin: 0;
-  padding: 0;
-  min-width: 160px;
-  display: flex;
-  flex-direction: column;
-  gap: 2px;
-}
-
-.lang-item {
-  display: flex;
-  align-items: center;
-  gap: 10px;
-  width: 100%;
-  padding: 8px 12px;
-  border: none;
-  border-radius: 8px;
-  background: transparent;
-  color: inherit;
-  font: inherit;
-  text-align: start;
-  cursor: pointer;
-  transition: background-color 0.15s, color 0.15s;
-}
-
-.lang-item:hover,
-.lang-item:focus-visible {
-  background-color: rgba(99, 102, 241, 0.12);
-  outline: none;
-}
-
-.lang-item.is-active {
-  color: var(--color-accent);
-  font-weight: 600;
-}
-
-.lang-item-icon {
-  font-size: 16px;
-  line-height: 1;
-}

frontend/src/pages/login/LoginPage.tsx

@@ -6,13 +6,18 @@ import {
   Form,
   Input,
   Layout,
+  Menu,
   Popover,
+  Space,
   Spin,
   message,
 } from 'antd';
 import {
   KeyOutlined,
   LockOutlined,
+  MoonFilled,
+  MoonOutlined,
+  SunOutlined,
   TranslationOutlined,
   UserOutlined,
 } from '@ant-design/icons';
@@ -105,26 +110,20 @@ export default function LoginPage() {
     return classes.join(' ');
   }, [isDark, isUltra]);
 
-  const langList = useMemo(
+  const langMenuItems = useMemo(
-    () => LanguageManager.supportedLanguages as { value: string; name: string; icon: string }[],
+    () => (LanguageManager.supportedLanguages as { value: string; name: string; icon: string }[]).map((l) => ({
+      key: l.value,
+      label: (
+        <Space size={8}>
+          <span aria-hidden="true">{l.icon}</span>
+          <span>{l.name}</span>
+        </Space>
+      ),
+    })),
     [],
   );
 
-  const themeIcon = !isDark ? (
+  const themeIcon = !isDark ? <SunOutlined /> : !isUltra ? <MoonOutlined /> : <MoonFilled />;
-    <svg viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth={2} strokeLinecap="round" strokeLinejoin="round" aria-hidden="true">
-      <circle cx="12" cy="12" r="4" />
-      <path d="M12 2v2M12 20v2M4.93 4.93l1.41 1.41M17.66 17.66l1.41 1.41M2 12h2M20 12h2M6.34 17.66l-1.41 1.41M19.07 4.93l-1.41 1.41" />
-    </svg>
-  ) : !isUltra ? (
-    <svg viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth={2} strokeLinecap="round" strokeLinejoin="round" aria-hidden="true">
-      <path d="M21 12.79A9 9 0 1 1 11.21 3 7 7 0 0 0 21 12.79z" />
-    </svg>
-  ) : (
-    <svg viewBox="0 0 24 24" fill="currentColor" stroke="currentColor" strokeWidth={1.5} strokeLinecap="round" strokeLinejoin="round" aria-hidden="true">
-      <path d="M21 12.79A9 9 0 1 1 11.21 3 7 7 0 0 0 21 12.79z" />
-      <path fill="none" d="M19 3l0.7 1.4 1.4 0.7-1.4 0.7L19 7.2l-0.7-1.4-1.4-0.7 1.4-0.7z" />
-    </svg>
-  );
 
   return (
     <ConfigProvider theme={antdThemeConfig}>
@@ -132,35 +131,30 @@ export default function LoginPage() {
       <Layout className={pageClass}>
         <Layout.Content className="login-content">
           <div className="login-toolbar">
-            <button
+            <Button
-              type="button"
               id="login-theme-cycle"
-              className="theme-cycle"
+              shape="circle"
+              size="large"
+              className="toolbar-btn"
               aria-label={t('menu.theme')}
               title={t('menu.theme')}
+              icon={themeIcon}
               onClick={cycleTheme}
-            >
+            />
-              {themeIcon}
-            </button>
             <Popover
               rootClassName={isDark ? 'dark' : 'light'}
               placement="bottomRight"
               trigger="click"
+              styles={{ content: { padding: 4 } }}
               content={
-                <ul className="lang-list">
+                <Menu
-                  {langList.map((l) => (
+                  mode="vertical"
-                    <li key={l.value}>
+                  selectable
-                      <button
+                  selectedKeys={[lang]}
-                        type="button"
+                  items={langMenuItems}
-                        className={`lang-item${lang === l.value ? ' is-active' : ''}`}
+                  onClick={({ key }) => onLangChange(key)}
-                        onClick={() => onLangChange(l.value)}
+                  style={{ border: 'none', minWidth: 160 }}
-                      >
+                />
-                        <span className="lang-item-icon" aria-hidden="true">{l.icon}</span>
-                        <span className="lang-item-name">{l.name}</span>
-                      </button>
-                    </li>
-                  ))}
-                </ul>
               }
             >
               <Button