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>

backend/version.py

@@ -1,3 +1,3 @@
 """Single source of truth for the StackPilot release version."""
 
-APP_VERSION = "0.37.6"
+APP_VERSION = "0.37.7"

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",

frontend/src/pages/Files.tsx

@@ -189,6 +189,7 @@ export function Files() {
       let ok = 0;
       let failed = 0;
       let doneBytes = 0; // bytes of fully-completed files
+      let firstError = ""; // surfaced so failures aren't silent
       // Show the bar immediately — small files may finish before the browser
       // emits any upload-progress event, so don't wait for the first one.
       setProgress({
@@ -213,15 +214,16 @@ export function Files() {
             });
           });
           ok += 1;
-        } catch {
+        } catch (err) {
           failed += 1;
+          if (!firstError) firstError = `${rel}: ${apiErrorMessage(err)}`;
         }
         doneBytes += f.size;
       }
-      return { ok, failed };
+      return { ok, failed, firstError };
     },
-    onSuccess: ({ ok, failed }, files) => {
-      if (failed) toast.warning(`Uploaded ${ok} file(s), ${failed} failed`);
+    onSuccess: ({ ok, failed, firstError }, files) => {
+      if (failed) toast.error(`Uploaded ${ok} file(s), ${failed} failed — ${firstError}`);
       else toast.success(`Uploaded ${ok} file(s)`);
       revealIfHidden(
         ...files.map((f) => (f as File & { webkitRelativePath?: string }).webkitRelativePath || f.name),
@@ -368,7 +370,12 @@ export function Files() {
                 multiple
                 onChange={(e) => {
                   const files = Array.from(e.target.files ?? []);
-                  if (files.length) uploadFolder.mutate(files);
+                  if (!files.length) {
+                    toast.error("Folder selection returned no files (nothing to upload).");
+                    return;
+                  }
+                  toast.info(`Starting folder upload: ${files.length} file(s)…`);
+                  uploadFolder.mutate(files);
                 }}
               />
             </>