feat: Decode battery temperature from the battery characteristic

[gwhillhouse]

The battery characteristic (fc540007) is 5 bytes: percent, charging flag, a little-endian uint16 battery temperature in °C×100 (bytes 2–3, same encoding as the other temperatures), and a legacy voltage byte. BatteryInfo.from_bytes only read the first two.

This adds BatteryInfo.temperature (°C, None when the field is zero so mugs that don't report it keep the old behaviour), carries it across the charger push events so a CHARGER_CONNECTED/DISCONNECTED event doesn't wipe it, and appends it to str(BatteryInfo).

Verified on an Ember Mug 2 (14 oz, firmware 406): 06 01 f8 11 00 → 6 %, on base, 46.00 °C. That reading is also the practical reason to expose it: a mug holding a hot drink on the coaster sits at the same percentage for an hour because the pack is above the ~45 °C charging cutoff, and the temperature makes that visible instead of looking like a dead battery. orlopau's notes document the same bytes: https://github.com/orlopau/ember-mug/blob/master/docs/battery.md

• pytest: 177 passed
• ruff check / ruff format --check: clean
• No change to existing as_dict() keys other than the new temperature field; existing 2-byte test still passes unchanged apart from asserting temperature is None.

I left CHANGELOG.md alone since you seem to write it at release time; happy to add a line if you'd prefer.

[gwhillhouse]

Context for where this came from, and the statistics-stream decode, is in #97.

[sopelj]

Awesome! Thanks for adding this. Good work! I'll test it on my mug and then create a release.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.79%. Comparing base (1bf20c1) to head (f30e22e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #96      +/-   ##
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+ Coverage   91.73%   91.79%   +0.05%     
==========================================
  Files          11       11              
  Lines        1029     1036       +7     
  Branches      127      129       +2     
==========================================
+ Hits          944      951       +7     
  Misses         64       64              
  Partials       21       21              

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