feat(theme): 管理后台新增模板选择器，支持运行时动态切换主题

新增功能：
- 主题发现模块：扫描 templates/ 目录下含 theme.json 的目录
- 站点设置新增 site.theme 键，DB 持久化 + 内存缓存
- 运行时切换主题：保存后即时重载 Gin 模板 + 静态资源哈希
- 管理后台 '外观主题' 页面：列表展示、选择、一键切换
- 动态静态资源服务：根据当前主题目录提供 CSS/JS

修复：
- account.html 残留孤儿 {{end}} 模板语法错误
- 动态静态资源 handler 误判 Gin *filepath 路径前缀为绝对路径

变更文件（12 modified + 3 new）：
- cmd/server/main.go: 动态主题加载 + 运行时重载 + 动态静态服务
- internal/theme/discovery.go: 新增主题扫描
- internal/config/site_settings.go: ThemeName()
- internal/service/site_setting_service.go: ThemeReloader + UpdateTheme
- internal/controller/admin/: ISP 扩展 + GetThemes/UpdateTheme/ThemePage
- internal/controller/auth_controller.go: 准则路径动态化
- internal/router/: InjectThemeReloader + 新路由
- templates/: 外观主题页面 + JS + CSS + 子导航

templates/admin/static/js/site-settings-theme.js

@@ -0,0 +1,109 @@
+/* =====================================================
+   MetaLab 管理面板 - 主题切换
+   立即生效，无需重启
+   ===================================================== */
+
+(function() {
+    'use strict';
+
+    var selector = document.getElementById('theme-selector');
+    var saveBtn = document.getElementById('theme-save-btn');
+    var infoEl = document.getElementById('theme-info');
+    var statusSection = document.getElementById('theme-status');
+    var statusMsg = document.getElementById('theme-status-msg');
+    var themes = [];
+    var currentTheme = '';
+
+    async function loadThemes() {
+        try {
+            var res = await api.get('/api/admin/site-settings/themes');
+            if (!res.success) {
+                showToast('加载主题列表失败', 'error');
+                return;
+            }
+            themes = res.data.themes || [];
+            currentTheme = res.data.current || 'MetaLab-2026';
+
+            // 填充下拉
+            selector.innerHTML = '';
+            themes.forEach(function(t) {
+                var opt = document.createElement('option');
+                opt.value = t.dir;
+                opt.textContent = t.display_name || t.name;
+                if (t.dir === currentTheme) {
+                    opt.selected = true;
+                    updateInfo(t);
+                }
+                selector.appendChild(opt);
+            });
+
+            saveBtn.disabled = false;
+            saveBtn.textContent = '保存';
+
+            // 选择变化时更新信息
+            selector.addEventListener('change', function() {
+                var selected = selector.value;
+                var found = themes.find(function(t) { return t.dir === selected; });
+                if (found) {
+                    updateInfo(found);
+                }
+                saveBtn.disabled = (selected === currentTheme);
+            });
+
+            // 保存
+            saveBtn.addEventListener('click', function() {
+                applyTheme(selector.value);
+            });
+        } catch (e) {
+            showToast('加载主题列表失败', 'error');
+        }
+    }
+
+    function updateInfo(theme) {
+        var parts = [];
+        if (theme.display_name) parts.push(theme.display_name);
+        if (theme.version) parts.push('v' + theme.version);
+        if (theme.author) parts.push('作者：' + theme.author);
+        if (theme.description) parts.push(theme.description);
+        infoEl.innerHTML = '<span class="setting-value">' + escapeHtml(parts.join(' &middot; ')) + '</span>';
+    }
+
+    async function applyTheme(themeDir) {
+        saveBtn.disabled = true;
+        saveBtn.textContent = '切换中...';
+        showStatus('正在切换主题...', '');
+
+        try {
+            var res = await api.put('/api/admin/site-settings/theme', { theme: themeDir });
+            if (res.success) {
+                currentTheme = themeDir;
+                saveBtn.textContent = '已生效';
+                showToast('主题已切换，即时生效', 'success');
+                showStatus('✅ 主题已切换为：<strong>' + themeDir + '</strong>，所有访客可见新主题', 'success');
+            } else {
+                saveBtn.disabled = false;
+                saveBtn.textContent = '保存';
+                showToast(res.message || '切换失败', 'error');
+                showStatus('❌ 切换失败：' + (res.message || '未知错误'), 'error');
+            }
+        } catch (e) {
+            saveBtn.disabled = false;
+            saveBtn.textContent = '保存';
+            showToast('切换失败', 'error');
+            showStatus('❌ 切换失败，请重试', 'error');
+        }
+    }
+
+    function showStatus(msg, type) {
+        statusSection.style.display = 'block';
+        statusSection.className = 'settings-section';
+        if (type === 'success') {
+            statusSection.classList.add('status-success');
+        } else if (type === 'error') {
+            statusSection.classList.add('status-error');
+        }
+        statusMsg.innerHTML = msg;
+    }
+
+    loadThemes();
+})();