feat: implement Files page with persistent storage and version tracking

Three-panel file manager (nav, list, details) modeled after Stirling-PDF.
- Backend: user_files table, /api/v1/files/* CRUD routes, file storage
  helpers, thumbnail generation via Sharp, recursive CTE version chains
- Frontend: FilesNav, FileList, FileDetails, FileUploadArea components,
  Zustand store, mobile layout with bottom sheet
- Integration: tool-factory auto-saves results as new versions when
  fileId is provided, "Open File" loads file into tool processing flow
- Search, bulk select/delete/download, version badges, tool chain tags

apps/api/src/lib/file-storage.ts

@@ -1,6 +1,6 @@
-import { mkdir, writeFile, unlink } from "node:fs/promises";
-import { join, extname } from "node:path";
 import { randomUUID } from "node:crypto";
+import { mkdir, unlink, writeFile } from "node:fs/promises";
+import { extname, join } from "node:path";
 import { env } from "../config.js";
 
 let storageReady = false;