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>

assets/admin/src/schema/sections/global/layout.js

@@ -61,7 +61,7 @@ export const layoutSchema = {
     theme_skin: 'f04494',
     dark_theme_skin_custom: '',
     close_backdrop: false,
-    theme_main_radius: 12,
+    theme_main_radius: 8,
     container_max_width: 1200,
     sidebar_enabled: true,
     sidebar_position: 'right',
@@ -147,7 +147,7 @@ export const layoutSchema = {
         },
         { 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' },
       ],
     },
     {