hub: S2 /offsite registry page (read-only) + nav + WGPeer.CreatedAt

Endpoint card + peers table (truncated pubkeys with full-value title attr,
bound peers link to /hosts/<id>); Offsite nav link in all 9 page templates;
render tests for endpoint/peers, empty, and not-configured states.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PSK5g6qYLknKj8u3QAFEr6

hub/internal/web/templates/customer_unified.html

@@ -17,6 +17,7 @@
                 <a href="/configs" class="nav-link active">Customers</a>
                 <a href="/apps" class="nav-link">Apps</a>
                 <a href="/hosts" class="nav-link">Hosts</a>
+                <a href="/offsite" class="nav-link">Offsite</a>
                 <a href="/configuration" class="nav-link">Configuration</a>
             </nav>
             <a href="/configs" class="back-link">&larr; All Customers</a>