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/templates/base.html

@@ -17,7 +17,7 @@
             </a>
             {% if channel_configured %}
             <span class="badge bg-success bg-opacity-25 small" style="padding-left: 0.35rem;" title="Private Channel Key Fingerprint: Messages encoded on this server use this shared channel key for additional encryption. Recipients must have the same channel key to decode.">
-                <i class="bi bi-shield-lock me-2" style="color: #6ee7b7;"></i><code style="font-size: 0.7rem; font-weight: 300; color: #f0c674;">{{ channel_fingerprint }}</code>
+                <i class="bi bi-shield-lock me-2" style="color: #6ee7b7;"></i><code style="font-size: 0.7rem; font-weight: 300; color: #c9a860;">{{ channel_fingerprint }}</code>
             </span>
             {% else %}
             <span class="badge bg-secondary bg-opacity-25 small text-muted" style="padding-left: 0.35rem;" title="Public Channel: No shared channel key configured. Messages use only passphrase and PIN for encryption.">