feat: Add area support for maintenance tasks

[Seidlm]

Summary

Adds the ability to assign a Home Assistant Area to each maintenance task. When a task has an area assigned, the corresponding binary_sensor entity is automatically placed in that area.

Changes

Backend

• store.py: Added area_id field to HomeMaintenanceTask, entity registry area update on add() and update_task()
• binary_sensor.py: Sets area_id via entity registry in async_added_to_hass()
• websocket.py: Added area_id to add_task schema and task constructor

Frontend

• types.ts: Added area_id to Task interface
• main.ts: Added area selector to new task form (advanced section) and edit dialog, area_id handling in add/save/edit handlers
• Translations: Added Area/Bereich labels in en.json and de.json

How it works

• New optional area dropdown in the task creation form (under Advanced)
• Area can also be set/changed when editing a task
• The area_id is persisted in storage and applied to the entity via the HA entity registry
• Fully backward compatible - existing tasks without area continue to work as before