feat(tasks): allow changing a task's project from the detail sheet

[paryanineil]

Summary

Adds a Project selector to the task detail sheet, so a task can be moved to a different project inline. Previously a task's project was fixed once it was created.

Behaviour

• New Project field (a Hive Project link picker) near the top of the detail sheet, above Milestone.
• Changing the project clears Milestone and Depends On — those reference records scoped to the previous project, and the backend doesn't auto-clear them, so leaving them set would point the task at a milestone/task from a different project.
• The Milestone and Depends On pickers now filter by the currently selected project, so their options stay consistent after a switch.
• Read-only for client users, consistent with the other fields.

Notes

• Frontend-only. Reuses the existing LinkField component and the standard useFrappeUpdateDoc autosave path — no backend/API or DocType changes.
• Assignees are intentionally left untouched (they're user-based via _assign, not project-scoped).

Testing

• yarn build and ESLint clean on the changed file.
• Verified in-browser: opened a task in one project, switched it to another via the new field — the change persisted and its milestone/depends-on were cleared (confirmed via the REST API).

[paryanineil]

Pushed a small follow-up commit that also touches e2e/tests/lock-due-date.spec.ts.

The new Project field surfaces the project's title in the detail sheet, and that spec located the Due Date field with .grid.gap-2 filtered by hasText: "Due Date". Because the test's fixture project is named "E2E Lock Due Date", the Project field's value now also matched that substring, so the locator resolved to two elements (strict-mode violation).

The fix targets the field by its exact "Due Date" label (filter({ has: page.getByText("Due Date", { exact: true }) })) instead of a substring, so it's robust to other fields' displayed content. Ran the full lock-due-date spec locally against this branch — all 6 pass.

(The other red E2E checks — project-types, task-pinning, and the tasks detail-sheet cases — are the pre-existing flakes that also fail on develop.)