21st-ui-build: Implement project-aware interfaces

# Build project-aware UI Preserve the product's identity while using 21st supply to accelerate implementation. ## Workflow 1. Read `.21st/design.json` and `.21st/DESIGN.md`. If neither exists, run `21st init --design-context`. 2. Inspect the target route, its closest components, project instructions, and existing tokens. Treat explicit project facts as stronger than generic taste. 3. Search before generating: ```bash 21st search "<specific UI need>" --context auto ``` 4. Reuse installed project primitives first. Retrieve or install a 21st result only when it reduces duplication or materially improves the result. 5. If no result fits, check MCP `get_usage.aiGenerationEnabled` or run `21st usage` and read the `21st AI generation` line. CLI 1.17.1+ also supports `21st usage --json`. Only when AI is explicitly enabled, generate project-aware alternatives using available AI credits: ```bash 21st generate "<goal and constraints>" --context auto --variants 3 ``` Otherwise, implement alternatives with your own coding agent, grounded in `21st search` and `21st get` (MCP: `search` and `get_component`). Do not call or suggest hosted `generate` or `iterate_generation` without AI access. 6. Implement the selected direction in the project's real framework. Do not paste sketch HTML when the project expects React components, tokens, or established APIs. 7. Preserve responsive behavior, keyboard interaction, visible focus, semantics, reduced motion, loading states, empty states, and error states. 8. Run the project's checks and `21st review <changed paths>`. Apply only safe, deterministic fixes automatically. ## Design rules - Prefer project tokens over new arbitrary values. - Prefer existing components over parallel lookalikes. - Preserve established density, radii, typography, iconography, and layout rhythm unless the user explicitly requests a redesign. - Use real product copy and states when available. - Avoid decorative gradients, excessive cards, glass effects, huge headlines, and animation unless supported by the project or requested. - Never claim a component, token, or capability exists without inspecting it. ## Handoff Report the direction used, reused 21st/project components, files changed, and verification performed. Record a durable visual choice in `.21st/design.json` under `decisions` when the user selects a new direction.