fix(auth): align the social-login surfaces with the reference theme

The "other login methods" block drifted from zibll on three axes. Gitee was
mapped to fa-code-branch because FA5 Free ships no Gitee brand glyph, so the
button showed a generic branch icon; Google showed the monochrome FA glyph
instead of the reference's multi-colour mark. The modal also printed an
implicit agreement line under the social buttons, which the reference guest
card does not have. And the top-nav user popover built its own
.social-login-divider / .oauth-row / .oauth-btn markup and then wrapped that
in a second separator and .social_loginbar, so the block rendered nested and
unlike the reference.

aksrt_brand_icon() now routes zibsvg-/customsvg- values through the shared
icon renderer, and the brand map resolves gitee to zibsvg-gitee and google to
zibsvg-google-color. Both sprite keys already exist in the runtime symbol
sheet, which the loader pulls in on every frontend page. Because
get_auth_providers_data() and binding_row() both go through
aksrt_brand_icon(), one map change fixes the icon on the modal, the user card,
the top-nav popover, the user-center binding list and the profile social
links.

render_social_buttons() is rewritten to the reference shape: an optional
.social-separator, a .social_loginbar, and one .social-login-item <type>
toggle-radius anchor per enabled provider with rel="nofollow" and a
"<label>登录" title. oauth_button_lg switches to the button-lg variant with a
label, and the WeChat entry carries qrcode-signin so auth.js opens the QR
panel instead of navigating, matching the reference. The navbar and the link
apply modal now append the block verbatim instead of re-wrapping it, and the
user-card loop adopts the same item attributes. The retired
.social-login-divider / .oauth-row / .oauth-btn rules are dropped from
base-1.css, and the binding-list icon rule gains an svg selector so the
sprite icons keep the 22px the FA glyphs had.

Verification: node --test across widget-system, navigation, admin,
asset-ledger, dev, front-submit and visual (613 tests), plus php -l on every
changed PHP file. Extended the auth source contract with the agreement-line,
brand-icon and popover-structure cases, and pinned the user-card guest
markup.

assets/css/routes/user-center/settings.css

@@ -265,7 +265,8 @@
 .aksrt-account-list--social .binding-item { border: 0; }
 .aksrt-account-list--social .aksrt-account-row { display: grid; grid-template-columns: 28px minmax(0, 1fr) auto auto; text-decoration: none; }
 .aksrt-account-list--social .binding-item__icon { display: grid; place-items: center; width: 24px; height: 24px; color: var(--color-ink, #333); }
-.aksrt-account-list--social .binding-item__icon i { font-size: 22px; line-height: 1; }
+.aksrt-account-list--social .binding-item__icon i,
+.aksrt-account-list--social .binding-item__icon svg { font-size: 22px; line-height: 1; }
 .aksrt-account-list--social .binding-status { display: inline-flex; align-items: center; gap: 4px; color: var(--color-success, #16a34a); font-size: 11px; white-space: nowrap; }
 .aksrt-account-list--social .binding-status--idle { color: var(--muted-2-color, var(--color-ink-4, #999)); }
 .aksrt-account-list--social .binding-unbind-btn { color: var(--this-color, #e4a11b); }