Feature Request: Add global settings config entry

## Summary

Device Tools currently has no global configuration — every config entry is a standalone modification. A dedicated Settings entry is needed as a central place for integration-wide options.

## Proposed Solution

Introduce a special `SETTINGS` modification type that is automatically created on first setup and cannot be deleted by the user. It appears in the integration UI like any other config entry but exposes integration-wide options via its options flow.

### Behavior

- Created automatically when the integration is first set up (or on migration if not present)
- Marked internally so it cannot be removed accidentally
- Holds global options that apply to all modifications
- Future global settings can be added here without architectural changes

### Initial options

None yet — this issue is a prerequisite for #65 (auto-labeling) and potentially other global settings in the future.

## Motivation

Prerequisite for any feature that requires a global toggle or setting across all Device Tools modifications.