PR 3: Observability back-links and terminology cleanup

- Added an Observability back-link at the top of all 7 Observability-area pages (AuditLog, SystemLogs, SystemInfo, AiUsageReport, UsageQuota, BannedIps, Diagnostics/Index) consistent with the Maintenance back-link pattern from PR 2
- Renamed company-level nav label and view title from "Notification Log" to "Email & SMS Log" to distinguish it clearly from the platform-level "Platform Notifications", "Audit Log", and "System Logs" surfaces

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/PowderCoating.Web/Views/Shared/_Layout.cshtml

@@ -1491,7 +1491,7 @@
                                     <li><a class="dropdown-item" asp-controller="Tools" asp-action="Index"><i class="bi bi-wrench-adjustable me-2"></i>Tools</a></li>
                                     <li><hr class="dropdown-divider"></li>
                                 }
-                                <li><a class="dropdown-item" asp-controller="NotificationLogs" asp-action="Index"><i class="bi bi-bell me-2"></i>Notification Log</a></li>
+                                <li><a class="dropdown-item" asp-controller="NotificationLogs" asp-action="Index"><i class="bi bi-bell me-2"></i>Email &amp; SMS Log</a></li>
                                 <li><a class="dropdown-item" asp-controller="BugReport" asp-action="Submit"><i class="bi bi-bug me-2"></i>Report a Bug</a></li>
                             </ul>
                         </div>