chore: configure dependabot updates

[Copilot]

Summary

• add Dependabot configuration for Dart/pub dependencies
• add weekly grouped updates for GitHub Actions, npm package directories, and devcontainer dependencies
• use semantic commit prefixes so Dependabot PRs satisfy PR title linting

Validation

• ./bin/mise run check
• parallel_validation

[Copilot]

Pull request overview

Adds a Dependabot configuration to automate dependency updates across the Flutter/Dart app and the repo’s supporting tooling (GitHub Actions, npm subprojects, and devcontainer). It also updates the mise lockfiles to include missing platform checksums/URLs, improving tool install reproducibility.

Changes:

• Added .github/dependabot.yml with weekly, grouped updates for pub, GitHub Actions, multiple npm directories, and devcontainers, using semantic commit prefixes.
• Updated mise.lock to include checksums for Node.js musl platform builds.
• Expanded mise.dev.lock with platform-specific download URLs and checksums for watchexec.

Reviewed changes

Copilot reviewed 1 out of 3 changed files in this pull request and generated no comments.

File	Description
mise.lock	Adds missing musl checksums for pinned Node.js tool downloads to improve integrity verification.
mise.dev.lock	Records platform-specific watchexec artifacts and checksums for reproducible dev-tool installs.
.github/dependabot.yml	Introduces scheduled, grouped Dependabot updates across Dart, Actions, npm workspaces, and devcontainer dependencies.