Group timeline rows by project and show folded spans

[Dreamfire78]

What

Closes #258, closes #180.

Projects on the timeline (#258). When the timeline shows several projects, each project now heads its own tasks with a row of its own:

• a collapse toggle that folds the project down to that single row
• the project's color and title (click opens the project)
• an add button that creates a task in that project (unfolding it first, so the new task is visible)

Since every task now sits under its project's row, the project chip on each task row is gone from the timeline. Table and board keep it. With a single project nothing changes.

How far folded work reaches (#180). A collapsed task draws a line out of each end of its bar to the earliest start and the latest end among its subtasks, capped by a tick, and only on a side the subtasks actually reach past. A collapsed task without dates of its own (the case in #180) gets the line across the whole span. A project row always shows its span the same way, since it has no bar of its own. Hovering the line shows the dates.

Details:

• The fold state lives in a new collapsedProjectRows setting (project paths), cleaned up with the other per-project state and carried along when a project is renamed or moved.
• "Collapse all / Expand all" on the timeline also folds the project rows; in the table it still covers tasks only.
• Dependency arrows and milestone lines now take each task's row index explicitly (GanttRow.row), as project rows sit between tasks.
• The recurrence mark and the label fields beside a bar move past the line's end so the line doesn't run through them.
• The line keeps clear of the bar's drag handles and link dots.
• New string gantt.spanTooltip, translated in all locales.

How to check

1. Open a timeline over two or more projects (project switcher → pick several, or "All projects").
2. Each project has its own row above its tasks. Fold one with its chevron: its tasks disappear and the line across its span stays.
3. Give a task subtasks that run past its own dates, then fold it: a capped line runs from the bar out to where the subtasks end (and start, if earlier).
4. Fold a task that has no dates but dated subtasks: the line spans the subtasks.
5. Hover a project row and click its +: the new task lands in that project.
6. Open a single project: no project row, same as before.

Checklist

• pnpm check, pnpm check:submission and pnpm test pass (tsc, oxlint, eslint --max-warnings 0 and vitest run clean locally; oxfmt --check is clean on the changed files)
• One change per PR
• CHANGELOG.md has an entry under Unreleased if users will notice the change
• The styleguide gallery is updated if a UI component changed (no shared component changed)

🤖 Generated with Claude Code

[Dreamfire78]

This screenshot shows the timeline markers for open and collapsed projects, as well as for individual collapsed tasks with subtasks

Add task button to projects