feat: prototype dynamic development proxy dashboard

[vitormattos]

This PR improves the local development experience around the shared Docker proxy.

What developers get

• https://localhost now shows the active development environments and their real URLs, grouped by Compose project.
• Unknown *.localhost URLs show a friendly 404 page with the active routes and a closest-host suggestion.
• The page shows environment checks for Docker Engine and runc, including a warning for the old runtime combination that can break container shutdown.
• Long Compose project names get a small usability hint with an example of using a shorter COMPOSE_PROJECT_NAME without renaming the directory.
• Multiple checkouts can share the same proxy. Each checkout keeps its own Compose network and the proxy connects to the active project networks. Stopping one checkout keeps the proxy running for the others; stopping the last one removes the shared proxy and certificate companion.
• Normal docker compose up still uses published images only; no local proxy image build is required.

Dashboard

Unknown hostname

Dependency and CI improvements

Third-party images used by the default development path are pinned by version and digest. Dependabot is configured to propose image updates through normal PRs and CI.

The proxy behavior is covered by ShellCheck, Bats unit/policy tests, and Docker integration tests for routing, friendly 404s, runtime diagnostics, shutdown, and multiple concurrent environments.