Task Reassignment Overhaul

The task picker now understands your survey structure:

• Survey-aware selection — Only see tasks that belong to the current survey
• Two-level paths — Properly assign parent→child task combinations for hierarchical surveys
• Works with all survey types — TopTask surveys show their configured task hierarchy; Discovery surveys let you assign any organization task
• Context required — Bulk actions on "All Responses" require a survey filter to ensure accurate assignments

These changes prevent accidental mis-assignments and ensure your task statistics stay accurate.