feat: Add starter categories for grouped starters

[EyalAmitay]

Add optional StarterCategory support to group starters visually in the UI. Categories appear as clickable buttons with optional icons, revealing their starters when selected.

Usage

@cl.set_starter_categories
async def starter_categories(user):
    return [
        cl.StarterCategory(
            label="Creative",
            icon="https://example.com/creative.png",  # optional
            starters=[
                cl.Starter(label="Write a poem", message="Write a poem"),
                cl.Starter(label="Write a story", message="Write a story"),
            ],
        ),
        cl.StarterCategory(
            label="Educational",
            starters=[
                cl.Starter(label="Explain", message="Explain quantum computing"),
            ],
        ),
    ]

Changes

• Backend: Added StarterCategory dataclass with label, icon (optional), and starters list
• Backend: Added @set_starter_categories callback decorator
• TypeScript: Added IStarterCategory interface in react-client
• Frontend: Added StarterCategory component, updated Starters.tsx to render category buttons
• Tests: Added backend unit test and Cypress E2E tests

Backward Compatibility

Fully backward compatible - existing @set_starters callbacks work unchanged and render flat starters as before. The starterCategories feature is additive and independent. When both defined, starterCategories will take precedence.

Test plan

• Backend UTs pass
• Cypress E2Es pass
• Verified behavior locally

Screen.Recording.2026-01-20.at.2.17.42.PM.mov

[cubic-dev-ai]

No issues found across 6 files

[cubic-dev-ai]

No issues found across 12 files