Add server-side dismiss persistence and SuperAdmin onboarding progress page

Progress widget dismiss now POSTs to Dashboard/DismissProgressWidget, writing
GuidedActivationDismissedAt to the DB so the widget stays hidden across devices
and cache clears (localStorage alone wasn't enough). BuildShopProgressWidgetAsync
suppresses the widget server-side when AllDone + dismissed.

New SuperAdmin page at /OnboardingProgress shows the activation funnel across
all tenant companies: wizard status, chosen path, milestone progress bar, key
dates (first job/quote, first invoice, workflow completed, widget dismissed),
and a status badge (Not Started / In Progress / Complete / Dismissed). Nav link
added under Users & Activity in the Platform Management sidebar.

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

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

@@ -1215,6 +1215,10 @@
                     </a>
 
                     <div class="nav-section-title">Users &amp; Activity</div>
+                    <a asp-controller="OnboardingProgress" asp-action="Index" class="nav-link">
+                        <i class="bi bi-rocket-takeoff"></i>
+                        <span>Onboarding Progress</span>
+                    </a>
                     <a asp-controller="PlatformUsers" asp-action="Index" class="nav-link">
                         <i class="bi bi-people-fill"></i>
                         <span>Platform Users</span>