Add schema YAML editor to CRUD UI

[eviltester]

Summary

Adds the Issue 46 schema YAML editing story to the standalone CRUD UI.

• adds a separate /schema Schema Edit page beside the Workspace view
• adds stateless schema preview endpoints for YAML-to-draft and draft validation
• adds tree/detail schema editing for model metadata, entities, fields, validations, object fields, and relationships
• adds canonical YAML, Mermaid, and Graphviz generation/export support
• adds Mermaid ER rendering with hide/show, zoom, horizontal/vertical layout toggle, resize handle, and relationship key
• adds schema dirty-state tracking and prompts before Workspace navigation, reset, refresh, or close
• keeps schema preview isolated from the live workspace schema/data
• adds a simple CRUD UI favicon

Closes #46

Validation

• node --check thingifier-crud-ui\src\main\resources\public\assets\app.js
• mvn -pl thingifier-crud-ui -am test
• mvn -pl thingifier-crud-ui -DskipTests checkstyle:check@project-fqn-check
• mvn -pl thingifier-crud-ui -am -DskipTests package

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Keep form handlers attached to the active draft

When a draft preview returns while the user is still editing a schema field, this replaces state.schemaDraft with a new object graph without re-rendering the editor. The existing inputs' input handlers still mutate the old entity/field objects captured by the detail renderers, so any subsequent keystrokes in that same control are omitted from later preview/export requests and can be lost in the downloaded YAML. Either keep the client draft object as the source of truth or re-render/rebind the editor when swapping it.

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