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/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 {