[Feature] Group monitors into collapsible sections (like Uptime Kuma)

### Summary
Add the ability to group monitors into logical sections on both the dashboard and status page.

### Problem
With many monitors, it becomes difficult to organize and navigate them. Users want to visually group related monitors together (e.g., all VMs in one section, websites in another).

### Proposed Solution
Implement monitor groupings similar to Uptime Kuma:

- Create named groups (e.g., "Docker Containers", "Websites", "External Services")
- Assign monitors to groups
- Display groups as collapsible sections
- Keep groupings separate from tags (they serve different purposes)
- Mockup Reference
- Uptime Kuma's implementation:

Uptime Kuma Grouping