fix(header): stop the nav midline chip rule from collapsing the quick-login buttons

The top-nav guest popover rendered its quick-login buttons as 29px circles
with zero gap: github, gitee and qq were touching. The user-card widget, which
uses the same .social_loginbar markup, was fine at 32px with 10px gaps.

header-userbox.css carries an AKSRT-specific rule, absent from the reference
theme, that gives every .toggle-radius inside .navbar-form a 29px midline
circle:

  .navbar-top .navbar-form .toggle-radius { flex: 0 0 29px; margin: 0; }

It exists for the account-row .badg chips, the theme toggle and the language
toggle. The popover social buttons are .toggle-radius too and also sit inside
.navbar-form, so they were caught by it. Two independent mechanisms broke the
geometry: flex-basis outranks width in a flex container, so flex: 0 0 29px beat
the .social_loginbar .social-login-item width: 32px !important; and the rule
(0,3,0) specificity outranked the (0,2,0) margin: 3px 5px.

Exempt the social buttons with :not(). The chips, the theme toggle and the
language toggle still match, and the popover buttons fall back to the
.social_loginbar geometry ported 1:1 from the reference.

Verified on the live site by measuring computed styles in headless Edge: the
popover buttons go from 29px/0px margins/0px gap to 32px/5px margins/10px gap,
the user-card widget is unchanged, and the account-row chips keep 29px (theme
toggle) and 36px (.badg). Rebuilt the public bundle so the exemption survives
the build, and contract-locked the selector plus the bundle copy.

Verification: node --test across widget-system, navigation, admin,
asset-ledger, dev, front-submit and visual (615 tests).

assets/css/build/public.css

@@ -2781,7 +2781,13 @@ main.container,
   object-fit: cover;
 }
 
-.navbar-top .navbar-form .toggle-radius {
+/* The account-row chips, the theme and the language toggles all want the 29px
+ * midline circle. The popover's quick-login buttons are .toggle-radius too and
+ * also live inside .navbar-form, but they must keep the .social_loginbar
+ * geometry from main.css: in a flex container flex-basis outranks width, so
+ * without this exemption the buttons collapse to 29px and lose the 5px side
+ * margins that space them apart. */
+.navbar-top .navbar-form .toggle-radius:not(.social-login-item):not(.xh-social-item) {
   flex: 0 0 29px;
   margin: 0;
   vertical-align: middle;