feat(sandbox): add sandbox file API service layer

- Add types for sandbox file API (SandboxFileNode, SandboxFileDownloadTicket)
- Add oRPC contracts for listFiles and downloadFile endpoints
- Add TanStack Query hooks (useGetSandboxFiles, useDownloadSandboxFile)
- Add useSandboxFilesTree hook with flat-to-tree conversion

web/contract/console/sandbox-file.ts

@@ -0,0 +1,30 @@
+import type {
+  SandboxFileDownloadRequest,
+  SandboxFileDownloadTicket,
+  SandboxFileListQuery,
+  SandboxFileNode,
+} from '@/types/sandbox-file'
+import { type } from '@orpc/contract'
+import { base } from '../base'
+
+export const listFilesContract = base
+  .route({
+    path: '/sandboxes/{sandboxId}/files',
+    method: 'GET',
+  })
+  .input(type<{
+    params: { sandboxId: string }
+    query?: SandboxFileListQuery
+  }>())
+  .output(type<SandboxFileNode[]>())
+
+export const downloadFileContract = base
+  .route({
+    path: '/sandboxes/{sandboxId}/files/download',
+    method: 'POST',
+  })
+  .input(type<{
+    params: { sandboxId: string }
+    body: SandboxFileDownloadRequest
+  }>())
+  .output(type<SandboxFileDownloadTicket>())