0.37.6: remember the "Show hidden" toggle across reloads

Uploaded dotfiles (.env) were persisting fine, but "Show hidden" is
component state that reset to off on every reload — so after refreshing,
hidden files disappeared from view and looked lost. Persist the toggle in
localStorage (sp.files.showHidden) so it survives reloads; combined with
0.37.5's auto-reveal, an uploaded .env now stays visible.

Frontend-only; all 3 images rebuilt+pushed 0.37.6.

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

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.37.5",
+  "version": "0.37.6",
   "type": "module",
   "scripts": {
     "dev": "vite",