Feat: add local Docker dev environment for the plugin

[Rom1-B]

Checklist before requesting a review

Please delete options that are not relevant.

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
• This change requires a documentation update.

Description

• It fixes #N/A
• Adds a .dev/ Compose stack and Makefile targets (make up, make down, make config) that spin up a local Metabase instance alongside GLPI, seeded with a dev admin account and static embedding enabled, plus the connection values needed to fill in the plugin's Setup > Metabase config page. This makes it possible to exercise the dashboard-embedding feature locally without a hand-provisioned Metabase instance.

Screenshots (if appropriate):