Add weekly heating schedule with per-slot control

[reloadman]

Summary

• Added a top-level heatingSchedule settings model with:
  • 7 days, 4 intervals per day
  • 15-minute time steps
  • per-interval enabled flag
  • actions: off, on, set target temperature
• Implemented runtime schedule execution in MainTask:
  • selects the active interval by local time
  • skips disabled intervals
  • supports previous-day carry-over for overnight behavior
  • applies manual override (Dashboard/MQTT) until the next active interval
• Updated localization for all supported languages (EN/RU/NL/IT/CN) for new schedule UI text.
• Updated API serialization/deserialization:
  • new top-level heatingSchedule JSON format
  • backward compatibility for legacy heating.schedule
• Increased settings payload limits for API/MQTT to support larger schedule JSON.

Why

This adds native timer-based heating automation with flexible per-slot behavior

Test plan

• Configure NTP server + timezone, save settings, verify local time is applied correctly.
• Enable schedule and set a slot (e.g. 15:30 -> On), verify heating switches on at the exact slot time.
• Set a slot to Off, verify heating is disabled at slot time.
• Set a slot to Target, verify heating is enabled and target temperature is applied.
• Disable a slot, verify it is skipped.
• Make a manual Dashboard/MQTT change during an active slot, verify override persists until the next active slot.
• Edit with All days, then switch to individual days and verify values are propagated correctly.

[Laxilef]

Hi @reloadman,

Thank you for your contribution! I need some time to study this.
One question: how is this better than the schedules in Home Assistant?

P.S. We already use NTP to update the date/time on the ESP:

OTGateway/src/Settings.h

Lines 39 to 42 in 4775d36

	struct {
	char server[49] = "pool.ntp.org";
	char timezone[49] = "UTC0";
	} ntp;