chore(llm): make system prompt opinionated for React + Tailwind

[Misacripto]

Why

Ensure generated UI projects follow a single, high-quality visual and technical standard (React + Vite, Tailwind CSS, lucide-react). This reduces variance in model outputs and guarantees the generated code is runnable in the WebContainer with consistent visuals similar to Lovable.

What

• Adds mandatory stack and design rules to app/lib/.server/llm/prompts.ts (getSystemPrompt): requires React (Vite), Tailwind CSS, lucide-react, modular components, tailwind/postcss configs, and an explicit install step in artifacts.
• No other runtime or parser logic was changed; this only tightens the system prompt the LLM receives.

Approach

The change embeds strict, machine-readable instructions inside the system prompt so streamed model responses will include the expected files (package.json, tailwind.config.js, postcss.config.js, src/index.css), modular React components, and an explicit <boltAction type="shell"> install command. This keeps generation centralized and avoids ad-hoc post-processing.

Notes for reviewers / caveats

• The change affects all LLM-generated artifacts that rely on this system prompt. Verify phrasing for any unintended strictness (for example, when user explicitly requests a different stack).
• Prefer pnpm when a pnpm lockfile exists; otherwise npm is used. Confirm this logic in integration tests if present.
• No behavior changes to the streaming parser or action runner are included here; those remain backward compatible.

Files changed

• app/lib/.server/llm/prompts.ts (system prompt updated)

Testing

• Trigger a model generation that creates a UI project and confirm the output includes the package.json, Tailwind/PostCSS configs, src/index.css, modular components, and an install shell action as required.

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