4. Add shared playground UI and JSON editor

[anish-sahoo]

• shared tabs, status badges, codemirror json editor, and playground styles
• includes colocated component/editor unit tests
• cog-playground stack: 1. Add Go playground server and handlers #3109, 2. Add playground frontend scaffold and early CI #3108, 3. Add core playground types and utilities #3114, 4. Add shared playground UI and JSON editor #3110/5. Add Cog playground API client #3106, 6. Add playground connection feature #3105/7. Add playground input controls and validation #3111/8. Add playground prediction controls and output #3107, 9. Wire up Cog playground and documentation #3113, 10. Compose playground app and add browser e2e #3112

[ask-bonk]

I'm Bonk, and I've done a quick review of your PR.

One issue found in the JSON editor tests and configuration related to keyboard accessibility.

[ask-bonk]

I'm Bonk, and I've done a quick review of your PR.

1. Summary: Adds shared playground UI components (segmented tabs, status badges, CodeMirror JSON editor) and responsive styles with colocated tests.

2. Issues:

   • High (test failure / accessibility): JsonEditor.test.tsx:144 asserts fireEvent.keyDown(...).toBe(true), but fireEvent returns an Event object (not a boolean), so the test will always fail. Additionally, CodeMirror's defaultKeymap traps the Tab key for indentation, which contradicts the test's stated goal of leaving Tab available for keyboard navigation. I've posted suggestions on the PR for both the test assertion and the keymap configuration.

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