feat(fridge): standardise fridge interface

• Introduce FridgeApplication as a base class that handles all report loop publishing and boilerplate for HAL implementation Custom fridge backends no longer need to implement these themselves (This change is fully backwards compatible)
• Rename the "system_monitor" package and service names to "fridge". Update dashboard widgets, schemas, dev container config, docs, and tests to the new fridge module paths, and default the docs/dev container to the application_class flow (init_module remains documented as a backwards-compatible option). There's a backwards compatibility layer built in here, that will be removed soon(See !494 (merged)).
• Closes #410 (closed)

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com