Improve UX, security, and maintainability

- Introduced a DevExpress toast-based notification system using `DxToastProvider` and `IToastNotificationService` for consistent feedback.
- Standardized request-to-domain mapping with AutoMapper/profile conventions or reflection-based mappers, enforced at the Core library level.
- Removed dynamic SQL string composition and migrated to parameterized queries to address SQL injection vulnerabilities. Added CI guardrails to prevent reintroduction.
- Refined UX on `EnvelopeSenderPage.razor` to enforce deterministic row double-click behavior and align row action buttons/tooltips.
- Enabled SPA-style language switching for immediate UI culture updates without page reloads.
- Added sender-scoped caching for "recent receiver suggestions" with short TTL, invalidation hooks, and cross-sender isolation.

AGENTS.md

@@ -281,6 +281,7 @@ Manual testing workflow:
 - P1 security workstream: remove dynamic SQL string composition/interpolation from active save/create/update paths (`string.Format`, `$"...{input}..."`, `ToSqlParam`) and migrate to parameterized queries only; treat this as a potential SQL injection vulnerability (stability + security risk), execute repo-wide audit, and add CI guardrails to block reintroduction.
 - Alternative UX design (Server component): in `EnvelopeGenerator.Server/EnvelopeGenerator.Server/Components/Pages/EnvelopeSenderPage.razor`, enforce deterministic row double-click behavior as exactly one action (preview OR edit), and keep row action buttons/tooltips aligned with the same rule to eliminate ambiguous navigation.
 - Implement SPA-style language switching so the UI culture updates immediately without a full page reload.
+- Implement sender-scoped caching for "recent receiver suggestions" used during envelope creation (wizard/editor flows): cache key should include sender identity, support short TTL + invalidation hooks after successful envelope receiver writes, and prevent cross-sender suggestion leakage.
 
 ## Database