DEV: workflow node expression editor using CodeMirror

[renato]

Replaces the existing contenteditable` expression editor with a CodeMirror-based editor and adds a backend expression preview endpoint.

Expression editor

• Syntax highlighting for {{ }} expressions with embedded JavaScript (variables, properties, strings, numbers, operators)
• Autocompletion with ranked sections: $json, trigger, $vars, $site_settings, $current_user, $execution, $('Node Name') references, and JavaScript globals (Math, JSON, Object, etc.)
• Type-aware method completions (string methods after a string value, array methods after an array, etc.)
• Hover tooltips showing variable descriptions and property types
• Argument info tooltips when the cursor is inside a function call
• Auto-close {{ }} braces with immediate completion trigger
• Drag-drop variable insertion from the context panel, context-aware (inserts bare variable inside {{ }}, wraps in {{ }} outside)
• Unclosed expression validation with error decoration
• Parent objects (e.g., post) are now draggable, not just leaf fields

Expression preview

• Real-time backend evaluation via POST /expressions/evaluate with debounced requests
• Builds evaluation context from declared node output schemas — works without any past execution data
• Overlays real values from the most recent successful execution when available
• Node-aware: $json reflects the upstream node's output, not just the trigger's
• Distinguishes undefined paths, invalid JavaScript, uncalled functions, and empty expressions with distinct labels and styling

Core CodeMirror infrastructure

• Generic codemirror-editor.gjs component with @extension callback for domain-specific behavior
• {{ }} Lezer grammar with mixed JavaScript parser, reusable for email templates, onebox templates, or any {{ }} surface
• JavaScript method completion data and syntax tree utilities as shared expression utils
• Single lazy loader to async load the CodeMirror editor