fix(l1): handle 402 seat_limit_exceeded on invite

Catches the structured detail from the seat-enforcement 402 and surfaces
a clear toast with current/limit counts instead of a silent failure.
Modal-with-upgrade-link is a v2 polish — Phase 1 just ships a toast.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/pages/AccountSettingsPage.tsx

@@ -237,8 +237,17 @@ export function AccountSettingsPage() {
       const invitesData = await accountsApi.getInvites()
       setInvites(invitesData)
     } catch (err) {
-      toast.error('Failed to send invitation')
-      console.error(err)
+      const resp = (err as any)?.response
+      if (resp?.status === 402 && resp?.data?.detail?.code === 'seat_limit_exceeded') {
+        const d = resp.data.detail
+        const label = d.role === 'l1_tech' ? 'L1' : 'Engineer'
+        toast.warning(
+          `${label} seats full: ${d.current}/${d.limit}. Upgrade your plan to add more.`,
+        )
+      } else {
+        toast.error('Failed to send invitation')
+        console.error(err)
+      }
     } finally {
       setIsInviting(false)
     }