Improve pattern discovery and build reliability in watch mode

[DomagojGojak]

• Added watchPatternDirectories — detects newly created pattern subdirectories at runtime, mirroring the existing component directory watcher with a corrected rebuild sequence (previews-only for referenced components, then targeted buildPatterns)
• Refactored watchComponentDirectories into a shared watchEntityDirectories factory, eliminating duplicated scaffolding that would otherwise be required to handle both components and patterns
• Fixed .handoff.ts files not being detected by the config-file gate (affected both components and patterns)
• Extended the build cache key in getComponentFilePaths to include pattern config files that reference a component, preventing stale cache hits when a pattern is added between sessions
• Graceful handling of empty/incomplete pattern config files during dev: downgraded from error to warn, skips the build attempt, and automatically retries when the file is saved with valid content
• Improved 404 handling for pattern pages — when a renamed pattern's stale URL is visited, the app now shows a properly styled 404 page inside the site layout instead of a bare red error text on a white screen. The 404 UI was extracted into a shared NotFound component, reused across both pattern and doc pages.