4.1.4: QR sharing, venv tarball, flash script improvements

QR Channel Key Sharing:
- Admin-only QR generator in about.html (was visible to all)
- QR button for saved keys on account page
- Fixed about() route missing channel status vars (bug)

Pi Build Optimization:
- Pre-built venv tarball support (39MB zstd, skips 20+ min compile)
- setup.sh auto-detects and extracts tarball if present
- Strip __pycache__/tests before tarball (295MB → 208MB)

Flash Script Improvements:
- flash-image.sh now uses config.json for headless WiFi setup
- Consistent wipe prompt on both flash scripts
- pull-image.sh re-enables auto-expand before shrinking

Build Docs:
- Added zstd and jq to pre-setup apt-get
- Documented fast build option with pre-built venv

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

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

frontends/web/templates/about.html

@@ -331,10 +331,12 @@
                 </div>
                 {% endif %}
 
-                <!-- Channel Key QR Generator -->
+                <!-- Channel Key QR Generator (Admin only) -->
+                {% if is_admin %}
                 <div class="card bg-dark border-secondary">
                     <div class="card-header">
                         <i class="bi bi-qr-code me-2"></i>Share Channel Key via QR
+                        <span class="badge bg-warning text-dark ms-2"><i class="bi bi-shield-check me-1"></i>Admin</span>
                     </div>
                     <div class="card-body">
                         <p class="small text-muted mb-3">Generate a QR code to share a channel key with others.</p>
@@ -366,6 +368,7 @@
                         </div>
                     </div>
                 </div>
+                {% endif %}
             </div>
         </div>