v1.1.6: battery voltage history by charge level, sub-watt tray power

[nojackno2-ctrl]

Records the battery driver's measured terminal voltage each time the integer battery percentage changes, and plots the resulting 0–100% curve on a new Battery Voltage History tab.

What's in it

• Per-percentage average / min / max voltage, change-event count and last-recorded time, persisted in LocalAppData and shown in an exact-value table beside the chart, with a clear action.
• Unobserved percentages are left as gaps rather than interpolated — an unmeasured voltage is never drawn as if it had been read.
• A voltage is only recorded when IsVoltageMeasured is true, so the internal nominal voltage used for power math never reaches the curve.
• Tray icon fix: every rate was rounded to an integer, so a real 0.1–0.9 W flow rendered as 0. Sub-watt rates now keep one decimal place; the integer and 99+ rules above 1 W are unchanged.

Review fixes applied before this PR

Four presentation defects in the new tab, none affecting recorded measurements:

1. Vertical gridlines were drawn across the whole container while the curve is plotted into an inset area — they now span the plot area, so a gridline sits under the percentage it marks.
2. The 25% / 75% X-axis labels sat at 30% / 70% (five equal columns with edge-aligned ends); the end columns are now 0.5*.
3. LastRecordedAt was bound raw, rendering a culture-dependent timestamp; a LastRecordedAtText property now matches the app's yyyy-MM-dd HH:mm:ss convention.
4. XAML placeholder text and the samples-column header disagreed with the localization strings that overwrite them at runtime.

Verification

• Debug and Release solution builds: -warnaserror, 0 warnings / 0 errors
• dotnet test: 17/17 passing (adds BatteryVoltageHistoryPointTests, TrayIconPowerFormattingTests)
• git diff --check: clean

Not exercised: the live 0–100% curve needs a full discharge cycle on real hardware to fill in.

🤖 Generated with Claude Code