fix(battery): round capacity instead of truncating it - #5398

Fixes #5392.

The capacity computed from charge_now/charge_full or energy_now/energy_full used integer division, so it was truncated rather than rounded. With full-at that error gets scaled up. With the numbers from the issue (energy_now 45420000, energy_full 57120000):

• the real charge is 79.52%, which was truncated to 79 (so the module showed 79%, not 80%)
• with full-at: 80: 79 * 100 / 80 = 98.75, which shows as 99% and never reaches 100%

Rounding with std::lround gives 80, which shows 80% without full-at and 100% with full-at: 80.

Testing

• Built and ran meson test --suite waybar in a container from Dockerfiles/debian (arm64): build OK, 3/3 tests pass, no warnings in battery.cpp
• Checked the old and new expressions with the values from the issue: 79% / 99% before, 80% / 100% after
• clang-format --dry-run --Werror is clean
• Not tested on real battery hardware