fix(l1): replace any casts with structural error types (eslint)

Frontend CI failed on @typescript-eslint/no-explicit-any in three L1
post-review fix sites. Replace `(err as any).response...` with the
codebase's established structural cast
`(err as { response?: { data?: { detail?: string } } })`, matching
TicketPickerModal / FolderEditModal / ProceduralEditorPage. The
AccountSettingsPage 402 handler gets the fuller seat-limit detail shape.

tsc clean, eslint clean.

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

frontend/src/components/l1/L1WalkTreeVariant.tsx

@@ -42,7 +42,7 @@ export function L1WalkTreeVariant({ session, onSessionUpdate, onDone }: Props) {
 
   const lastError = (err: unknown): string => {
     if (typeof err === 'object' && err && 'response' in err) {
-      const detail = (err as any).response?.data?.detail
+      const detail = (err as { response?: { data?: { detail?: string } } }).response?.data?.detail
       if (typeof detail === 'string') return detail
     }
     return 'Unexpected error'

frontend/src/pages/AccountSettingsPage.tsx

@@ -237,7 +237,12 @@ export function AccountSettingsPage() {
       const invitesData = await accountsApi.getInvites()
       setInvites(invitesData)
     } catch (err) {
-      const resp = (err as any)?.response
+      const resp = (err as {
+        response?: {
+          status?: number
+          data?: { detail?: { code?: string; role?: string; current?: number; limit?: number } }
+        }
+      }).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'

frontend/src/pages/l1/L1Dashboard.tsx

@@ -48,7 +48,7 @@ export default function L1Dashboard() {
       })
       navigate(`/l1/walk/${response.session_id}`)
     } catch (err) {
-      const detail = (err as any)?.response?.data?.detail
+      const detail = (err as { response?: { data?: { detail?: string } } }).response?.data?.detail
       const msg =
         typeof detail === 'string' ? detail : 'Failed to start walk. Try again.'
       toast.error(msg)