feat(tasks): add per-task kanban boards

[GatewayJ]

Summary

• render each root task as a selectable board with independent child-task cards
• add title/ID search, assignee filtering, dependency indicators, and responsive board controls
• simplify task-board actions and remove obsolete manual planning/start UI plumbing

Validation

• pnpm --dir web/app check
• pnpm --dir web/app build
• pnpm --dir web/app typecheck
• targeted TasksView, task-controller, task-model, and workspace-sidebar tests

Impact

Frontend task-board presentation and navigation only. Backend task lifecycle, parent-child dependencies, execution rooms, and sessions are unchanged.

[GatewayJ]