Fix/app kit integration without mcp

[ditadi]

Summary

• Make ServerPlugin independent of Databricks - middleware now only applies to plugins that declare needsRequestContext = true
• Add needsRequestContext property to Plugin base class (default: false)
• Remove required env vars from ServerPlugin (DATABRICKS_APP_PORT, FLASK_RUN_HOST have defaults)
• Auto-detect static path from common build directories (dist/, client/dist/, build/, public/, out/)

Test plan

• createApp({ plugins: [server()] }) should start without any Databricks env vars
• createApp({ plugins: [server(), analytics()] }) should apply Databricks middleware only to analytics routes
• Static files should be auto-detected and served from first directory containing index.html
• All existing tests pass