feat: create TaskItem composable with tests

[opencode-agent]

Done! Here's a summary of what was implemented:

TaskItem Composable

• Created at foliary/src/commonMain/kotlin/dev/appoutlet/foliary/core/ui/component/task/TaskItem.kt
• Wrapped in the existing FoliaryCard for consistent rounded-corner styling with border and shadow
• Checkbox on the start side using Checkbox(checked = isCompleted, onCheckedChange = null) — display-only, no interaction logic
• Title with single-line ellipsis; when isCompleted = true, the title gets TextDecoration.LineThrough and reduced opacity (0.38f)
• Description (optional) capped at maxLines = 3 with TextOverflow.Ellipsis
• Overdue pill (red, using errorContainer/onErrorContainer) shown only when isOverdue = true and isCompleted = false
• Includes @Preview variants for default, overdue, and completed states

String resources

• Added task_item_overdue to strings.xml

Compose UI tests

• Created at foliary/src/commonTest/kotlin/dev/appoutlet/foliary/core/ui/component/task/TaskItemTest.kt
• Tests cover:
  • Title and description rendering
  • Description hidden when null
  • Checkbox presence
  • Overdue pill shown only when overdue and not completed
  • Completed state rendering (checkbox + distinct title styling)

All 7 JVM tests pass successfully.

Closes #128

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