0.37.7: surface folder-upload diagnostics (find why it does nothing)

Folder upload still reported as doing nothing, and without browser access
the failure point is invisible. Make every outcome visible on-screen:

- onChange: if the folder picker returns 0 files, toast an error; otherwise
  toast "Starting folder upload: N file(s)…" so it's clear the upload fired
  (independent of the progress bar rendering).
- Per-file failures are no longer swallowed: capture the first error and
  show it in the result toast ("Uploaded X, Y failed — <path>: <reason>").

This pinpoints whether the picker returns nothing, the upload never starts,
or the requests fail (and why). Frontend-only; all 3 images pushed 0.37.7.

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

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.37.6",
+  "version": "0.37.7",
   "type": "module",
   "scripts": {
     "dev": "vite",