feat(admin): apply design system to Stops pages

- Stops list, new, and detail pages use new PageHeader with Operations
  eyebrow, Stops & Routes title, and editorial subtitle.
- StopsCalendarClient: replace hardcoded amber (#f59e0b) and brown (#92400e)
  draft markers with --admin-warning / --admin-warning-soft tokens. Calendar
  structure unchanged (drag/click/edit preserved).
- Add/Edit/StopDetail modals: replace hardcoded red rgba with --admin-danger-soft
  and color-mix derived borders.
- StopsLocationsTabs: swap hardcoded emerald-* for --admin-primary /
  --admin-primary-soft tokens.
- StopMessagingForm: align with design system (ha-field, ha-eyebrow, tokens).
- StopProductAssignment error surface: tokens instead of red rgba.
- StopsHeaderActions already tokenized via AdminButton; no change.

TS clean. Calendar functionality unchanged.

src/components/admin/AddStopModal.tsx

@@ -145,7 +145,7 @@ export default function AddStopModal({ isOpen, onClose, brandId, duplicateFrom,
           <div
             role="alert"
             className="flex items-start gap-2 rounded-lg px-3 py-2 text-xs text-[var(--admin-danger)]"
-            style={{ background: "rgba(220, 38, 38, 0.06)", border: "1px solid rgba(220, 38, 38, 0.15)" }}
+            style={{ background: "var(--admin-danger-soft)", border: "1px solid color-mix(in srgb, var(--admin-danger) 25%, transparent)" }}
           >
             <svg className="h-3.5 w-3.5 mt-0.5 shrink-0" viewBox="0 0 24 24" fill="none" stroke="currentColor" strokeWidth={2.5}>
               <circle cx="12" cy="12" r="10" />