fix(navigation): restore the reference 8px radius and unclip the mega menu

Traced against the running site (docker theme at AKSRT-theme) with real
computed styles:

- theme_main_radius defaulted to 12, so --main-radius was 12px and every
  dropdown panel, card and button was rounder than the reference, which
  sets 8px. Default is now 8 in the runtime reader and both schemas.
- The public bundle loads after main.min.css. Its bare img rule replaced
  the inline image baseline the menu relies on, and html{overflow-x} made
  the root a clipping context that crops the fixed senior mega menu.
  Reset is now scoped to content; the horizontal-overflow guard is on body.
- The senior mega menu renders a .container inside the dropdown where the
  reference keeps Bootstrap's 15px gutter; the padding-free site-shell
  .container rule stripped it, running the card/link grid to the edges.

Rebuilt public.css / public.min.css and the admin SPA.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/Admin/Schema/GlobalSchema.php

@@ -228,7 +228,7 @@ class GlobalSchema {
                             ['key' => 'theme_skin', 'label' => ' ', 'type' => 'palette', 'default' => 'f04494', 'dependsOn' => ['key' => 'theme_skin_custom', 'operator' => 'empty'], 'options' => self::skin_palette()],
                             ['key' => 'dark_theme_skin_custom', 'label' => '夜间模式主题色', 'type' => 'color', 'default' => '', 'help' => '在此可单独设置夜间模式的高亮色，清空此处则使用上方默认高亮色'],
                             ['key' => 'close_backdrop', 'label' => '全局关闭背景高斯模糊', 'type' => 'switch', 'default' => false, 'help' => '在支持的浏览器上部分UI(例如导航栏)会显示背景高斯模糊的特效，但此功能比较消耗浏览器性能。同时系统会自动检测用户浏览器性能情况，如果性能较差，将自动关闭此特效，以提供更流畅的体验'],
-                            ['key' => 'theme_main_radius', 'label' => '卡片圆角', 'type' => 'spinner', 'default' => 12, 'min' => 0, 'max' => 30, 'step' => 1, 'unit' => 'PX', 'description' => '页面卡片的圆角尺寸'],
+                            ['key' => 'theme_main_radius', 'label' => '卡片圆角', 'type' => 'spinner', 'default' => 8, 'min' => 0, 'max' => 30, 'step' => 1, 'unit' => 'PX', 'description' => '页面卡片的圆角尺寸'],
                         ]],
                         ['key' => 'layout', 'title' => '布局设置', 'fields' => [
                             // 1200–2200 与 Settings::container_max_width() 归一化区间一致。

src/Settings/ThemeSearch.php

@@ -408,9 +408,9 @@ class ThemeSearch {
         return array_values(array_intersect(array_map('sanitize_key', (array) $raw), ['pc_nav', 'm_menu']));
     }
 
-    /** 卡片圆角 theme_main_radius（0–30，默认 12）→ --main-radius。 */
+    /** 卡片圆角 theme_main_radius（0–30，默认 8）→ --main-radius。 */
     public static function main_radius(): int {
-        return max(0, min(30, (int) Settings::get_value('theme_main_radius', 12)));
+        return max(0, min(30, (int) Settings::get_value('theme_main_radius', 8)));
     }
 
     public static function close_backdrop(): bool {