Feature/update UI

[pikann]

Summary

• MCP server docs — Added a full MCP Server section to README.md documenting the @paca-ai/paca-mcp npm package: quick-start config for Claude Desktop and other MCP-compatible clients, environment variables reference, and a table of all available tool categories.
• Brand panel refresh — Updated login page feature cards: replaced "The P-A-C-A Cycle" with a Plugin Marketplace entry (new Puzzle icon), tightened the AI Agents card copy to emphasise sandboxed containers, and condensed the tagline.
• Actionable onboarding steps — Getting-Started step 1 is now a real <button> that opens the create-project dialog when the user has permission; subsequent steps render as locked (dimmed) until prerequisites are met.
• Disabled quick-action buttons — Quick-action items without an onClick handler are now visually disabled (opacity-50, cursor-default, no hover effects) instead of rendering as broken interactive elements.
• Exclude agent bot from user list — UserRepository.List now filters out the internal _paca_agent_bot account so it never appears in member pickers or admin user tables.

Type of Change

• Documentation
• UI / Frontend
• Backend fix

Checklist

• The change is focused and scoped.
• Related documentation is updated.
• New structure or direction is explained clearly.
• I avoided unnecessary detail or premature abstraction.