fix: follow established callback pattern for MonthlySpent sensor

[eMeF1]

Summary

Fixes the MonthlySpent sensor not updating by following the established callback pattern used by all other sensors in the codebase.

Problem

After PR #54, the MonthlySpent sensor was not updating because it used a custom async callback handler (_handle_coordinator_update) that didn't work with the synchronous callback system in RohlikAccount.publish_updates().

Solution

• Remove custom _handle_coordinator_update method
• Use async_write_ha_state directly as callback (matches all 40+ other sensors)
• Move processing logic to native_value property (lazy evaluation pattern)
• Follows the exact same pattern used by all other sensors in the codebase

Changes

• ✅ Removed async callback handler that was incompatible with sync callback system
• ✅ Changed callback registration to use async_write_ha_state directly
• ✅ Moved _check_and_reset_month() and _process_new_orders() calls to native_value property
• ✅ Updated async_will_remove_from_hass to remove correct callback

Testing

• ✅ Verified sensor updates correctly after fix
• ✅ Confirmed orders are processed when sensor value is read
• ✅ Tested state persistence and order deduplication still works
• 🔄 Additional real-world testing planned: Will validate with next actual order delivery

Related

Follow-up fix to #54 - addresses callback pattern issue discovered during testing.

[eMeF1]

@dvejsada, all tested and working as expected.