feat(stress): Y-axis + tap/scrub interaction on Today's Timeline chart

[ujix]

Summary

• Adds a Y-axis to the intraday stress line chart (labels 0–3 matching the stress scale) with a reserved 32dp left gutter so the axis never overlaps the line
• Tap anywhere on the chart to place a vertical crosshair and a floating pill tooltip showing stress level + hour (e.g. 1.7 @ 14:00)
• Drag / swipe left-right to scrub across the timeline; crosshair and tooltip track the finger in real time
• Crosshair dismisses on finger lift — no persistent state left in the UI
• Chart height increased from 72dp → 100dp to accommodate axis labels and give the line more breathing room
• X-axis ruler padded to align with the chart area (excludes the Y-axis gutter)

Implementation notes

• Touch handling uses awaitEachGesture + awaitFirstDown inside a pointerInput modifier — no external dependencies
• Tooltip value computed with integer-tenths arithmetic (avoids String.format)
• All drawing (Y-axis, line, crosshair, pill) is on a single Canvas — no extra composable layers

Test plan

• Open Stress screen on a day with HR/RR strap data → Today's Timeline should show Y-axis labels (0 / 1 / 2 / 3)
• Tap the chart → crosshair and tooltip appear at the tapped hour
• Drag left/right → crosshair tracks smoothly
• Lift finger → crosshair disappears
• Verify no regression on days with no daytime data (section remains hidden)