File browser: upload progress bar (0.21.5)

filesApi.upload now accepts an onProgress callback wired to axios
onUploadProgress; the Files page shows a progress bar with percentage
while uploading. Single-file upload tracks that file's bytes; folder
upload tracks overall progress across the N files (file i/N + current
file's bytes).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.21.4",
+  "version": "0.21.5",
   "type": "module",
   "scripts": {
     "dev": "vite",