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'