feat: add TreeViewItem component and Gallery samples

[Sanlorng]

Summary

• Add compact TreeViewItem overloads for leaf and expandable items.
• Support 16.dp hierarchy indentation, optional icons, standard selection indicators, and caller-owned checkbox slots.
• Animate checkbox-slot enter/exit and the horizontal content shift while preserving caller-owned checkbox state.
• Add standard, checkbox, and options Gallery samples backed by one shared nested TreeNode dataset.
• Register only visible tree rows through lazy intervals and animate item placement/removal without rebuilding the tree data.

PixPin_2026-08-10_11-46-09.mp4

Behavior

• A supplied checkbox slot replaces the standard selection indicator.
• Checkbox mode maps selected to checkbox state; indeterminate parent nodes remain unselected.
• Sample row clicks remain enabled no-ops when a checkbox is supplied.
• Expandable items expose a dedicated click region from the level/checkbox boundary to the icon or text boundary.
• Expansion interactions share the row interaction source, and click bounds plus collapsed chevrons respect RTL layout.
• The options sample exposes checkbox, icon, and enabled states.
• Every sample row uses animateItem() for expansion and collapse changes.

Review fixes

• Correct the expansion overlay's logical end padding from the measured physical content edge in both LTR and RTL.
• Mirror collapsed chevrons in RTL while keeping the expanded direction downward.
• Avoid registering hidden zero-height LazyColumn items; contiguous collapsed siblings stay in a single lazy interval, and recursion follows only visible expanded branches.

Validation

• ./gradlew :fluent:compileCommonMainKotlinMetadata :gallery:sharedApp:compileCommonMainKotlinMetadata
• git diff --check
• Final read-only source review: no Critical, Important, or Minor findings.

Gradle/build-script changes in the local checkout are intentionally excluded from this PR.

[Copilot]

Pull request overview

Adds a reusable hierarchical TreeViewItem component and Gallery demonstrations.

Changes:

• Adds leaf and expandable tree-item overloads.
• Supports indentation, icons, checkboxes, selection, and transitions.
• Adds standard, checkbox, and configurable Gallery samples.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
TreeViewItemScreen.kt	Adds tree datasets and interactive Gallery samples.
TreeViewItem.kt	Implements the new tree-item component and defaults.

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