Context

Issue #784 specified CodeMirror as the code editor for the Settings page JSON/YAML view. The initial implementation in PR #902 used a plain <textarea> as a pragmatic starting point. This follow-up replaces it with CodeMirror for a proper editing experience.

Scope

• Install CodeMirror 6 (@codemirror/view, @codemirror/state, @codemirror/lang-json, @codemirror/lang-yaml)
• Replace the <textarea> in web/src/pages/settings/CodeEditorPanel.tsx with a CodeMirror editor
• Add syntax highlighting for JSON and YAML
• Add line numbers, bracket matching, and error highlighting
• Ensure the editor respects the existing design tokens (colors, fonts, spacing)
• Update Storybook story for CodeEditorPanel

Not in scope

• Code folding, minimap, or other IDE-like features (keep it simple)

Blocked by

• feat: Settings page (search, dependency indicators, grouped rendering) #784 (must merge first)