Add theme loader

[remdui]

Summary

• What changed?

  • Implemented a complete theme system based on real theme folders in themes/.
  • Added strict theme discovery/validation (theme.toml + static/ required, metadata required).
  • Added dashboard theme settings with auto-detected selectable themes.
  • Added runtime theme static endpoint /theme-static/<path> with default-theme fallback for missing assets.
  • Added one-time warnings when selected themes are invalid/missing assets.
  • Removed legacy root static theme path usage (static/css/site.css deleted, templates now always use theme URL).
  • Updated Docker/Compose for theme mounts/env (MYLONITE_THEMES_ROOT, MYLONITE_THEMES_DIR).
  • Updated docs for theme architecture and custom theme authoring.
  • Added/updated tests for resolver logic, panel settings, route wiring, and theme static behavior.

• Why is this needed?

  • Enables user-created themes in a safe, predictable way.
  • Keeps templates fixed while allowing static-only visual customization.
  • Prevents broken styling by falling back to default theme assets.
  • Aligns runtime/container setup with the new theme-first architecture.
  • Removes old static CSS assumptions so there is a single source of truth.

Type of change

• Bug fix
• New feature
• Refactor
• Docs
• CI/CD
• Security

Checklist

• I ran the test suite locally (DJANGO_DEBUG=true python manage.py test)
• I ran quality checks locally (ruff check . and ruff format --check .)
• I updated tests for behavior changes
• I updated docs (README.md/CONTRIBUTING.md) where relevant
• I verified there are no unexpected schema migration changes
• I assessed security impact (secrets, auth, permissions, dependencies)