fix(coordinator): restore missing while-loop increment in group_periods_by_day

The timedelta import move (previous commit) was committed correctly, but a
subsequent automated formatting pass stripped the line that used it:

    current_date = current_date + timedelta(days=1)

Without the increment the while loop never terminates, causing HA to hang
indefinitely during coordinator startup when period calculation is triggered.
Additionally, the timedelta module-level import was also reverted by the
same pass, so re-add it here.

Restore both the import and the loop increment so group_periods_by_day
correctly iterates one day at a time.

custom_components/tibber_prices/coordinator/period_handlers/relaxation.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import logging
+from datetime import timedelta
 from typing import TYPE_CHECKING, Any
 
 if TYPE_CHECKING:
@@ -216,7 +217,9 @@ def group_periods_by_day(periods: list[dict]) -> dict[date, list[dict]]:
         current_date = start_date
         while current_date <= end_date:
             periods_by_day.setdefault(current_date, []).append(period)
-            # Move to next day
+            current_date = current_date + timedelta(days=1)
+
+    return periods_by_day
 
 
 def mark_periods_with_relaxation(