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/inbounds/QrPanel.vue

@@ -47,7 +47,7 @@ function download() {
     </div>
     <div v-if="showQr" class="qr-panel-canvas">
       <a-qrcode class="qr-code" :value="value" :size="size" type="svg" :bordered="false"
-        :title="t('copy')" @click="copy" />
+        color="#000000" bg-color="#ffffff" :title="t('copy')" @click="copy" />
     </div>
   </div>
 </template>