Add optional id field to widgets

[terzioglub]

Summary

• Adds optional id string field to Widget (model.go, json schema, TSX loader)
• Pure addition: no validator rules, no existing fields touched, no frontend changes
• Matches agent-runner's widget id (which is required there; we're keeping it optional in dac)
• Enables cloud's selection/deep-link/data-fetch flows that key off widget.id

Test plan

• `make test` green
• `make build` green
• All existing testdata/example dashboards still validate
• 5 new test cases covering YAML/JSON round-trip + omitempty + optionality

🤖 Generated with Claude Code