Shared document groups, viewer empty state, and testing attribute fixes

Consistent empty state when no file is associated: Clicking a cell that has no linked document now shows a clear message — "No file selected" (with a prompt to pick one from the panel below) or "No files uploaded to this datasource yet" (with an upload shortcut) — instead of auto-loading an unrelated file from the datasource.
Visual marker for rows without a matched document: In datasources where some rows didn't receive a matched document, the extraction cells for those rows now show a blue dash on a gray background, making it immediately clear that AI extraction won't run on that row.
Viewer panel width no longer jumps: The document viewer holds its width as you move between different cell types — no more resizing when switching from a match cell to a retrieval cell and back.

Section state remembered as you navigate: Collapsing a section (Answer, Reasoning, Activity, and others) now keeps it collapsed as you move between cells in the same session. Reloading the page resets everything back to fully expanded.
Tick mark cells always reachable: Cells that have a tick mark applied but no associated document now correctly show the Cell tab, so you can view or update the applied tick mark without it disappearing.

Per-task matching mode: When multiple workpapers share a document group, each workpaper can now independently choose AI matching or Custom Rules matching — changing the setting in one workpaper no longer affects the others.
Automatic ownership transfer: If the workpaper that originally created a shared document group stops using it, ownership automatically passes to another workpaper still using the group so it stays accessible with no disruption.
Switching groups preserves settings and workflow: When you replace an attached shared document group with a different one, the current workpaper's matching configuration carries over and any connected downstream datasources are automatically re-linked — the workflow diagram stays intact.
Full access for importing workpapers: Workpapers that share an existing document group can now comment on cells, change matching mode, and trigger runs — previously some of these actions silently applied to the original owner workpaper instead.
Workflow diagram arrows always visible: Parent-to-child arrows now appear correctly for all datasources regardless of which matching mode each one is configured with.

Collective testing attribute results corrected: Testing attribute columns set to Collective mode now correctly produce one shared pass/fail verdict for the entire record group rather than one result per record.
Testing attributes on document-only datasources: Testing attribute columns now run correctly on datasources that contain only uploaded files (with no parent table), producing one result row per uploaded file.