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>

README.md

@@ -124,6 +124,11 @@ For development or custom modifications:
 The application can be configured using either command-line arguments or environment variables. Environment variables
 take precedence over command-line defaults.
 
+> [!IMPORTANT]
+> The `M2M_TEMPERATURE_UNIT` option must match Home Assistant's unit system (Settings → General → Unit System)
+> so setpoints and readings are interpreted correctly. If mismatched, climate entities will show incorrect values (e.g.
+> 21°C treated as 21°F) and commands may result in unexpected temperatures.
+
 ### Required Configuration
 
 | CLI Option            | Environment Variable | Description                      |
@@ -151,6 +156,7 @@ take precedence over command-line defaults.
 | `-l, --log-level`         | `M2M_LOG_LEVEL`         | `info`         | Log level: `silent`, `fatal`, `error`, `warn`, `info`, `debug`, `trace` |
 | `-f, --log-format`        | `M2M_LOG_FORMAT`        | `pretty`       | Log format: `pretty`, `json`                                            |
 | `-s, --mysa-session-file` | `M2M_MYSA_SESSION_FILE` | `session.json` | Path to Mysa session file                                               |
+| `-t, --temperature-unit`  | `M2M_TEMPERATURE_UNIT`  | `C`            | Temperature unit (`C` = Celsius, `F` = Fahrenheit)                      |
 
 ## Usage Examples