Add SMS Agreements admin page and update help docs

- Add /SmsAgreements SuperAdmin page listing per-company SMS terms acceptance
  status, with stats cards, filter/search, and a full acceptance history modal
  (terms version, accepted by, timestamp, IP, user agent)
- Add SMS Agreements nav link under Tenants & Billing in the platform sidebar
- Update HelpKnowledgeBase and Help docs (Quotes, Settings) to document
  quote approval via SMS and the reuse of existing approval tokens

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

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

@@ -1199,6 +1199,10 @@
                         <i class="bi bi-lightning-charge"></i>
                         <span>Stripe Events</span>
                     </a>
+                    <a asp-controller="SmsAgreements" asp-action="Index" class="nav-link">
+                        <i class="bi bi-file-earmark-check"></i>
+                        <span>SMS Agreements</span>
+                    </a>
 
                     <div class="nav-section-title">Content &amp; Communication</div>
                     <a asp-controller="Announcements" asp-action="Index" class="nav-link">