fix(web): render login QR codes dark-on-light so scanners can read them

The QR images used theme-aware colours, so in the default dark theme they were
rendered light-on-dark (inverted). Many in-app scanners — notably the Kugou
music app — cannot decode an inverted QR, so the code looked fine on screen but
silently failed to scan. Force standard dark-on-light regardless of theme; the
white quiet-zone frames it cleanly in dark mode anyway. Affects all platforms'
QR login (netease/qq/bilibili/kugou).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/views/Settings.vue

@@ -969,13 +969,15 @@ async function startQrLogin(platform: string) {
     if (qrImg) {
       qr.dataUrl = qrImg;
     } else {
+      // QR codes must be dark-on-light to stay scannable. Do NOT invert the
+      // colours for the dark theme: many in-app scanners (notably the Kugou
+      // music app) cannot decode a light-on-dark QR, so a themed code looks
+      // fine on screen but silently fails to scan. The white quiet-zone frames
+      // it cleanly in dark mode anyway.
       qr.dataUrl = await QRCode.toDataURL(qrUrl, {
         width: 200,
         margin: 2,
-        color: {
-          dark: store.theme === 'dark' ? '#ffffff' : '#000000',
-          light: store.theme === 'dark' ? '#2a2a2a' : '#ffffff',
-        },
+        color: { dark: '#000000', light: '#ffffff' },
       });
     }