Add Android Compare chart value scrubbing

[kavemang]

Summary

• add tap-to-select and drag-to-scrub interaction to Android's Compare overlay chart
• show a crosshair, per-series point markers, and a floating tooltip with the selected date and each metric's real unit-aware value
• snap across the plotted date domain so sparse and irregular series select the genuinely nearest available day
• keep the existing normalized line geometry, correlation calculations, colors, and legend unchanged

Why

The Android Compare chart only drew static normalized lines. Users could see the overall shapes and min/max legend, but could not inspect the actual values for a specific day. The Apple Compare chart already provides this crosshair and multi-series value readout.

Tapping now keeps a day selected for inspection. Horizontal dragging scrubs across days and clears the transient selection when released. If a selected metric has no value on that day, the tooltip shows an em dash rather than inventing or carrying a value.

Validation

• ./gradlew :app:testFullDebugUnitTest --tests com.noop.ui.CompareChartSelectionTest
• ./gradlew :app:testFullDebugUnitTest
• python Tools/i18n_audit.py --ci upstream/main
• git diff --check

Addresses the remaining Android chart-value visualization gap in #300.