Document local Flutter verification commands for contributors

[SebTardif]

Problem

This repository does not currently document the expected local Flutter verification commands for contributors. In practice, contributors need Flutter installed with both flutter and dart available on PATH, and they need a short list of commands to run before opening a PR.

Why this matters

Missing local setup and verification guidance creates avoidable friction for contributors, especially for quick bug-fix PRs. It also increases the chance that changes are proposed without local formatting, analysis, or test runs.

Success criteria

• Add contributor-facing guidance in the repository docs
• Document that the Flutter SDK must be installed and flutter/dart should be available on PATH
• Include a minimal local verification command set for formatting, analysis, and tests