fw/services/battery: add optional 80% charge limit to extend battery lifespan

[spr4bhu]

This PR adds a software charge limit feature that stops charging at 80% and resumes at 77% (3% hysteresis) to reduce lithium battery degradation from sustained high charge levels. The feature is exposed as a user-facing toggle in System settings, off by default.

Changes:

• Add battery_charge_limit module that calls battery_set_charge_enable() to disable/re-enable charging based on battery percentage thresholds; includes a 60s rate limit to prevent rapid charger cycling and a 60s periodic timer to cover gaps in fuel gauge event delivery when charging is disabled

• Add charge_limit_enabled pref (persistent on/off setting via SettingsFile, default false) with getter/setter following the existing stationary_mode_enabled pattern

• Add "Charge Limit (80%)" toggle row in System settings menu, mirroring the existing "Stand-By Mode" toggle

• Hook battery_charge_limit_evaluate() into battery_monitor_handle_state_change_event() and battery_charge_limit_init() into battery_monitor_init()

• Register battery_charge_limit.c in services/battery/wscript_build

  Resolves Ability to limit battery charge to X% #722

[EspressoTini]

Amazing, i have been meaning to circle back on this. Thank you so much!

[ericmigi]

Awesome. Thanks for the PR! Need to add this behind one of the features that only enables the setting to be shown on the phone side. We're reducing how many settings are shown on the Watch UI and encouraging people to add the setting on the phone side.

@jplexer do you have an example that we can show to folks on how to add this on both sides?

[jplexer]

fw side it needs to be added to the settings blob db whitelist (like: 4d8abd9)

On the mobile side I do not know, @sjp4 definitely knows more there

[gmarull]

While plugged in, if not charging, the watch will run from USB power, so the battery will not drain. I guess you can simply disable the charger and forget about any hysteresis.

[spr4bhu]

Made the changes, thanks.

[spr4bhu]

@sjp4 can I have the format on how the features are to be added on phone side