feat(widget): focus overflowing task lists

[xz-dev]

Summary

• when a collapsed task list exceeds maxVisible, normally anchor its window at the first unfinished task in the configured sort order
• if fewer than maxVisible tasks remain after that focus point, backfill from earlier tasks so every oversized collapsed window still contains exactly maxVisible task rows
• show separate … N earlier and … N later markers, with the final window used when every task is complete
• follow Pi's global app.tools.expand state (Ctrl-O by default) without registering a competing shortcut or persisting temporary expansion state
• retain showAll behavior and keep hiddenAt as a legacy-compatible setting

Testing

• npx vitest run test/task-widget.test.ts (38 tests)
• npm run lint
• npm run typecheck
• npm test (193 tests)
• npm run build
• independent review: approved

Notes

• lists at or below maxVisible remain fully visible
• all three commits are GPG-signed and remain independent of the firework animation patch
• CHANGELOG.md and generated dist files are unchanged