Edit backup destinations and schedules in Settings (0.64.1)

Both had update endpoints, but Settings only offered add, delete and, for
schedules, enable/disable, so a changed NAS address or backup time meant
deleting and recreating the entry.

Each row now has an Edit button that swaps it for the prefilled form, as
for private registries.

- Destinations: name and connection fields. Stored secrets come back
  masked, so they start empty with "Stored - leave empty to keep it"; the
  server keeps the stored value for an empty one. The type is fixed.
- Schedules: destination, timing, retention, volumes, stop-first and
  enabled. The stack is fixed, since retention matches backups by stack
  id. The last status shows warning/retrying in amber instead of red.

Tests pin that an empty secret keeps the stored one, a typed one replaces
it, and a timing change moves next_run.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/package.json

@@ -1,7 +1,7 @@
 {
   "name": "stackpilot-frontend",
   "private": true,
-  "version": "0.64.0",
+  "version": "0.64.1",
   "type": "module",
   "scripts": {
     "dev": "vite",