fix ios line gradient crash and LineChart popup IOOBE

[NadeemIqbal]

Two independent fixes in LineChart, kept as separate commits so they can be reviewed and reverted independently.

1. iOS ClassCastException in drawLineGradient on CMP 1.11.0 (closes #167)

Gradient.kt was using the low-level Skia interop path: Paint().shader = LinearGradientShader(...) inside drawIntoCanvas { it.drawPath(p, paint) }. In CMP <= 1.10 the iOS native bridge happened to unwrap this, but CMP 1.11.0 changed the internal Shader representation, so the runtime downcast in the iOS paint.shader setter fails:

kotlin.ClassCastException: class androidx.compose.ui.graphics.Shader
  cannot be cast to class org.jetbrains.skia.Shader
  at kfun:...drawLineGradient + 1995

Fix: use the public, multiplatform-safe DrawScope.drawPath(path, brush) with Brush.verticalGradient. The original gradient was strictly vertical (Offset(0f, 0f) to Offset(0f, _size.height)), so the swap is 1:1 and removes the drawIntoCanvas escape hatch entirely. Function signature is unchanged, both call sites in LineChart.kt keep compiling.

2. LineChart.showPopup IOOBE on empty / stale line data (closes #146, closes #158)

showPopup read linesPathData[dataIndex] and xPositions[startIndex] without guarding against three cases:

1. A Line whose values list is empty produces empty xPositions; the index access throws IndexOutOfBoundsException.
2. LaunchedEffect(data, minValue, computedMaxValue) { linesPathData.clear() } empties the state list whenever data updates, but a pointer event mid-update still tries to read linesPathData[dataIndex], surfacing as SnapshotStateList.get IOOBE on the gesture thread. This is exactly the stack in 🐛 LineChart crash in showPopup (IndexOutOfBoundsException) even when caller guards empty data #158.
3. xPositions empty even when pathData is present.

Three early-return guards added before the index access. Each fails this iteration only, so a malformed line does not poison a sibling line.

Local build verification

./gradlew :compose-charts:compileKotlinIosArm64 \
          :compose-charts:compileKotlinIosSimulatorArm64 \
          :compose-charts:compileKotlinIosX64 \
          :compose-charts:compileKotlinDesktop
BUILD SUCCESSFUL

No new warnings introduced.

[TheNephilim88]

@ehsannarmani please review and merge, so the lib can be used with latest compose multiplatform versions

[ehsannarmani]

Hi guys.
Thank you for contributing @NadeemIqbal
Sorry, currently i have no time to review, but i will do it ASAP.