feat(history): render nested subtasks as recursive tree

[hannesrudolph]

Summary

Add recursive subtask rendering to the History UI so nested subtasks can be expanded and navigated at arbitrary depth using the existing visual pattern.

Changes

• Introduce a recursive subtask tree model (SubtaskTreeNode) and recursive counter utility (countAllSubtasks) in history types.
• Refactor grouped task construction to build nested subtask trees via buildSubtree while preserving root-level grouping and search behavior.
• Update history rendering to support nested expand/collapse and depth-based indentation in TaskGroupItem and SubtaskRow.
• Wire nested toggle handlers through history containers (HistoryView, HistoryPreview).
• Add a new unit test suite for nested row behavior (SubtaskRow.spec.tsx).
• Expand existing tests for grouped task tree construction and recursive subtask counts.
• Update task group tests to validate nested rendering and recursive counts.

Verification

• cd webview-ui && npx vitest run src/components/history/tests/useGroupedTasks.spec.ts src/components/history/tests/TaskGroupItem.spec.tsx src/components/history/tests/SubtaskRow.spec.tsx

[roomote]

Rooviewer   See task

Re-reviewed after f8ff70f. Both previously flagged issues are now resolved. No new issues found.

• HistoryView.tsx: Duplicate countAll function -- reuse the exported countAllSubtasks from types.ts instead of redefining it locally.
• SubtaskRow.tsx: max-h-[500px] on the nested children container may clip expanded content for nodes with many subtasks. Consider increasing to match the max-h-[2000px] used in TaskGroupItem.

Previous reviews

• 9547655: Review #1
• e7c6ac7: Review #2

_{Mention @roomote in a comment to request specific changes to this pull request or fix all unresolved issues.}