fix: Unable to change the set point when Home Assistant is configured with Fahrenheit temperature unit (#73)

# Fix Temperature Handling in Fahrenheit Mode

## Problem

When operating in **Fahrenheit mode**, Mysa still expects temperature
values to be provided in **Celsius**.
However, Home Assistant sends integer Fahrenheit values (e.g.,
`72.02°F`), which convert to **non-aligned Celsius values** like
`22.22°C`.

Mysa’s API only accepts temperature values that are either **whole
numbers** or **increments of 0.5°C** (for example: `21.0`, `21.5`,
`22.0`).
As a result, values such as `22.22°C` or `21.72°C` are considered
invalid and are **rejected** by Mysa’s API.

---

## Root Cause

- The original code accepted **0.1°C precision** and **0.5°C step
size**.
- When Home Assistant runs in Fahrenheit, the conversion from °F to °C
produces fractional values that are not valid (e.g., 72°F → 22.22°C).
- Because Mysa enforces strict 0.5°C increments, these fractional
setpoints caused failed updates.

---

## Solution

This update ensures valid behavior when using Fahrenheit mode **while
keeping the current behavior for Celsius**:

- Adds a new environment variable:  
  **`M2M_TEMP_UNIT`** — accepts either:
  - `C` *(default)*
  - `F` *(for Fahrenheit operation)*
- When running in Fahrenheit mode (`M2M_TEMP_UNIT=F`):
  - Celsius values are **rounded and clamped to the nearest 0.5°C**.  
  - Temperature step size and precision are adjusted:
    - Precision → `1°F`
    - Step size → `1°F`
- When running in Celsius mode, existing logic remains unchanged (0.1
precision, 0.5 step).

---

## Technical Summary

| Mode | Env Variable | Precision | Step | Conversion Behavior |
|------|---------------|------------|------|----------------------|
| Celsius | `M2M_TEMP_UNIT=C` (default) | 0.1°C | 0.5°C | Direct
pass-through |
| Fahrenheit | `M2M_TEMP_UNIT=F` | 1°F | 1°F | Convert °F → °C, snap to
0.5°C |

The rounding logic ensures that when a Fahrenheit value (e.g., `72°F`)
is converted to Celsius (`21.72°C`), it is adjusted to the nearest valid
half-degree (`21.5°C` or `22.0°C`).

# Demo


https://github.com/user-attachments/assets/bbffe5fe-a3be-43cb-aed0-f63bdfacb1d4

---------

Co-authored-by: Pascal Bourque <pascal@cosmos.moi>

src/thermostat.ts

@@ -51,8 +51,11 @@ export class Thermostat {
     private readonly mqttSettings: MqttSettings,
     private readonly logger: Logger,
     public readonly mysaDeviceFirmware?: FirmwareDevice,
-    public readonly mysaDeviceSerialNumber?: string
+    public readonly mysaDeviceSerialNumber?: string,
+    public readonly temperatureUnit?: 'C' | 'F'
   ) {
+    const is_celsius = (temperatureUnit ?? 'C') === 'C';
+
     this.mqttDevice = {
       identifiers: mysaDevice.Id,
       name: mysaDevice.Name,
@@ -81,9 +84,9 @@ export class Thermostat {
           min_temp: mysaDevice.MinSetpoint,
           max_temp: mysaDevice.MaxSetpoint,
           modes: ['off', 'heat'], // TODO: AC
-          precision: 0.1,
-          temp_step: 0.5,
-          temperature_unit: 'C', // TODO: Confirm that Mysa always works in C
+          precision: is_celsius ? 0.1 : 1.0,
+          temp_step: is_celsius ? 0.5 : 1.0,
+          temperature_unit: 'C',
           optimistic: true
         }
       },
@@ -118,7 +121,17 @@ export class Thermostat {
             if (message === '') {
               this.mysaApiClient.setDeviceState(this.mysaDevice.Id, undefined, undefined);
             } else {
-              this.mysaApiClient.setDeviceState(this.mysaDevice.Id, parseFloat(message), undefined);
+              let temperature = parseFloat(message);
+
+              if (!is_celsius) {
+                const snapHalfC = (c: number) => Math.round(c * 2) / 2;
+                const clamp = (v: number, min: number, max: number) => Math.min(max, Math.max(min, v));
+                // Snap to 0.5 °C and clamp to device limits
+                const setC = snapHalfC(temperature);
+                temperature = clamp(setC, this.mysaDevice.MinSetpoint ?? 0, this.mysaDevice.MaxSetpoint ?? 100);
+              }
+
+              this.mysaApiClient.setDeviceState(this.mysaDevice.Id, temperature, undefined);
             }
             break;
         }
@@ -137,7 +150,7 @@ export class Thermostat {
         device_class: 'temperature',
         state_class: 'measurement',
         unit_of_measurement: '°C',
-        suggested_display_precision: 1,
+        suggested_display_precision: is_celsius ? 0.1 : 0.0,
         force_update: true
       }
     });