Self-contained widget packages and localization for 0.5.0

[Misiu]

Summary

• discover self-contained widget packages from built-in and local widget directories
• move Calendar, Entity State, and Weather data providers into their owning packages
• scope providers per widget and support backend-only community widgets in Studio
• localize Weather from project language with package-owned EN/PL translations
• require strict semantic widget versions and align Integration/Renderer at 0.5.0

Validation

• 58 Python tests
• Ruff and mypy
• 26 frontend tests and production build
• external unknown-widget discovery/composition test
• exact Weather Liquid contract tests