feat: add multi-device provisioning and local remote-config UI

- add multi-device portal device management, key rotation, and sync request APIs
- add Pi settings UI for remote server URL, device name, and API key
- add local test-connection and sync-now actions
- make upload and config sync reuse dynamic shared credentials
- add Phase 25.5c migration and docs updates

Co-authored-by: Copilot <[email protected]>

modules/app_device_config.py

@@ -72,9 +72,18 @@ def update_device_config(body, errors):
     if "sync_enabled" in body:
         if not isinstance(body["sync_enabled"], bool):
             errors["sync_enabled"] = "Must be true or false."
+        elif body["sync_enabled"]:
+            from . import app_remote_config
+
+            remote = app_remote_config.get_remote_config()
+            if not remote["server_url"] or not remote["device_name"] or not remote["api_key_configured"]:
+                errors["sync_enabled"] = "Configure server URL, device name, and API key before enabling server sync."
         else:
             _set_sync_enabled(body["sync_enabled"])
 
+    if "sync_enabled" in body and body["sync_enabled"] and "sync_enabled" not in errors:
+        _set_sync_enabled(True)
+
     return errors