fix(admin): Seller Management UX review - a11y label fix, icon list, review doc

Reviewed the Phase 1 UI against every other Backoffice page. Found and
fixed 2 real issues; everything else verified already consistent
(built entirely from shared components, so hover/focus/dialog-a11y/
dark-readiness/contrast come from those components, not reinvented).

Fixed:
- Message textarea had no id/aria-describedby wiring (app-input
  self-wires this via injected FormFieldContext; the raw textarea -
  no dedicated textarea component exists yet - never got it, so the
  visible label's `for` pointed nowhere). Added explicit aria-label
  bound to the same translation key as the visible label.
- Learn More dialog's feature list would render native browser
  bullets (no global list-style reset exists outside details>summary
  in styles.scss). Replaced with checkCircle icon + text rows,
  consistent with how the rest of the app pairs icons with list/status
  meaning.

Added docs/architecture/foundation/Seller-Management-UX-Review.md
documenting both fixes plus everything checked and confirmed already
consistent (empty-state usage, icon reuse, translations completeness
across en/ru/hy, responsive at 1280px/375px, dialog a11y verified via
accessibility tree not assumed).

tsc --noEmit clean, arch:check (boundaries + cycles) clean. Live-
verified: Learn More dialog shows all 6 items each with an icon
(confirmed via DOM query), textarea aria-label confirmed
"Сообщение", no console errors.

docs/architecture/foundation/README.md

@@ -52,6 +52,7 @@ It is a platform runtime that must support unlimited tenants from one Angular ap
 - [ADR-011](adr/ADR-011-optional-seller-management-module.md) — decision record
 - [Seller-Management-Diagrams.md](Seller-Management-Diagrams.md) — hierarchy, bootstrap gate, type diagram
 - [Seller-Management-Domain-Models.md](Seller-Management-Domain-Models.md) — typed models, optional sellerId fields
+- [Seller-Management-UX-Review.md](Seller-Management-UX-Review.md) — UX/accessibility review of the Phase 1 UI
 
 ### Engineering Rule Documents