feat: 添加5套主题切换：深邃暗/纯净白/赛博朋克/莫兰迪/护眼绿

ui.js

@@ -282,6 +282,24 @@ saveSettingsBtn.addEventListener('click', async () => {
   }
 });
 
+// ── Theme switcher ─────────────────────────────────────────────
+
+function applyTheme(theme) {
+  document.documentElement.dataset.theme = theme;
+  localStorage.setItem('imgGen-theme', theme);
+  document.querySelectorAll('.theme-btn').forEach(btn =>
+    btn.classList.toggle('active', btn.dataset.theme === theme)
+  );
+}
+
+document.querySelectorAll('.theme-btn').forEach(btn => {
+  btn.addEventListener('click', () => applyTheme(btn.dataset.theme));
+});
+
+// Restore saved theme
+const savedTheme = localStorage.getItem('imgGen-theme');
+if (savedTheme) applyTheme(savedTheme);
+
 // ── Start ──────────────────────────────────────────────────────
 
 init();