Validate dashboard YAML before deploy (#60)

[JohanLorenzo]

A new widget with no id, or with id: null, used to fail with a generic "Failed to parse dashboard YAML" error. Dashboards now fail before any API call, and the message names the widget and says to write id: 0. Both inputs stay rejected, so a typo in an existing ID still surfaces as an error.

Validation deserializes into the existing dashboard structs, so each error carries the widget path. A second check reports every widget without content in one run.

Evidence, dashboards deploy repro with a one-widget dashboard:

Before:

⚠ Dashboard 'repro' failed to deploy: Failed to parse dashboard YAML

After, with no id:

⚠ Dashboard 'repro' failed to deploy: Failed to parse dashboards/0-repro.yaml: widgets[0].id: missing or null id (use `id: 0` for a new widget)

With id: 0, validation passes and the request reaches the API.

Fixes #60