Fix QR key loader: remove conflicting CSS, proper centering

- Remove duplicate qr-crop-container styles from encode/decode templates
- Use only qr-scan-container from style.css (flex centering + object-fit)
- Fix rsaQrSection to use align-items: center for horizontal centering
- Darken channel key fingerprint in header (#f0c674 → #c9a860)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontends/web/static/style.css

@@ -1253,7 +1253,8 @@ footer {
    ---------------------------------------------------------------------------- */
 #rsaQrSection {
     display: flex;
-    justify-content: center;
+    flex-direction: column;
+    align-items: center;
 }
 
 #rsaQrSection .drop-zone {