Dart coding conventions

• Prefer final over var where the value never changes.
• Use lowerCamelCase for variables and functions, UpperCamelCase for types.
• Keep functions small and focused; prefer pure functions where practical.
• Always handle errors explicitly; avoid swallowing exceptions silently.
• Add a doc comment (///) to every public declaration.
• Use dart format defaults (80-column width) for all files.
• Prefer collection literals and spread operators over imperative building.