Add cooling / ice-bath support (heat *and* cool)

[thorwaler]

Why

The controller is currently heating-only: it demands heat while water is below
target and treats water >= target as "satisfied". For ice baths / cold plunges the
goal is inverted — run the pump + aux output (a chiller) to cool down to a target
that sits below the current water temperature. Today the controller simply does
nothing when the water is too warm.

This PR makes heating/cooling a selectable, persisted hvac_mode on the climate
entity (heat / cool / off) and inverts the thermostat logic in cool.

What changed

All temperature decisions flow through one hysteresis helper plus two "target
reached" guards, so the change is small and localised.

const.py

• OPT_KEY_COOL_MODE = "cool_mode", DEFAULT_COOL_MODE = False.

coordinator.py

• Read self.cool_mode from options in __init__ (next to hvac_enabled).
• New async def set_cool_mode(self, cool) that persists to options, mirroring
  set_hvac_enabled.
• _thermostat_demand() is now direction-aware:

if getattr(self, "cool_mode", False):
    # Cooling: ON above (target+hot), OFF at/below (target-cold)
    if prev_on:
        return ct > (tt - cold)
    return ct > (tt + hot)
# Heating (unchanged)
if prev_on:
    return ct < (tt + hot)
return ct < (tt - cold)

• The two water_temp >= target_temp_effective → demand off guards (PV path and aux
  path) are direction-aware — in cool mode "reached" is water <= target.

climate.py

• Add HVACMode.COOL to _attr_hvac_modes.
• hvac_mode returns COOL when cool_mode is set; hvac_action returns
  HVACAction.COOLING when active in cool mode.
• async_set_hvac_mode: COOL → set_cool_mode(True) + enable; HEAT →
  set_cool_mode(False) + enable; OFF → disable.

sensor.py (minor, separate commit — drop if unwanted)

• Change the hardcoded chlor_spoons unit from "Löffel" to "spoons" so the dose
  suggestion reads in English.

Behaviour

• cool runs the pump + aux (chiller) when water is above target and stops with the
  same hysteresis once at/below it.
• PV / power-saving demand inverts cleanly too, so "cool on solar surplus" works.
• Heating behaviour is unchanged when cool_mode is off (the default).

One decision to confirm

I removed the start_manual_heat_to_target() call from the HEAT branch of
async_set_hvac_mode. As-is, selecting heat starts a bathing timer (manual
heat-to-target), which is surprising for a plain heat/cool toggle — it turned bathing
mode on every time I switched to heat. Removing it makes heat/cool symmetric
(continuous thermostat control). If you'd prefer to keep the manual-heat behaviour on
heat, happy to gate it so only the toggle path skips it.

Deliberately left heating-tuned (possible follow-ups)

• Frost protection still forces pump/main near freezing — a useful floor for ice
  baths, but it overrides cooling below the frost threshold.
• Boost and dynamic-target season/weather offsets remain heating-shaped.

Testing

With cool selected and target a few degrees below water temp: the aux (chiller)
switch fires, hvac_action reports cooling, and it cuts out once water drops past
target with the configured hysteresis. Heating is unchanged with cool_mode off.

[thorwaler]

Companion frontend PR: lweberru/pool_controller_dashboard_frontend#2 — adds the matching cooling display + heat/cool toggle to the card.