fix(qr): lock QR code modules to black-on-white across all themes

AntD's <a-qrcode> defaults the module color to the active theme's
text token. Under the dark and ultra-dark themes that text is a light
gray, so the QR rendered low-contrast on the white canvas background
and phones could not lock onto it. Pinned color="#000000" and
bg-color="#ffffff" on every <a-qrcode> usage (share links in
QrPanel, 2FA enrollment in TwoFactorModal, sub/json/clash codes on
SubPage) so the contrast stays high regardless of panel theme.

frontend/src/pages/settings/TwoFactorModal.vue

@@ -82,7 +82,7 @@ async function copyToken() {
       <p>{{ t('pages.settings.security.twoFactorModalFirstStep') }}</p>
       <div class="qr-wrap">
         <a-qrcode class="qr-code" :value="qrValue" :size="180" type="svg" :bordered="false"
-          error-level="L" :title="t('copy')" @click="copyToken" />
+          color="#000000" bg-color="#ffffff" error-level="L" :title="t('copy')" @click="copyToken" />
         <span class="qr-token">{{ token }}</span>
       </div>
       <a-divider />