Add a tree / tree-grid object view type for self-referencing hierarchies (business unit, org chart) #1885

Problem

Hierarchical, self-referencing objects (business unit / org chart, category trees, menu trees, BOM, nested comments) have no native view type to render them with indentation + expand/collapse. Today they can only be shown flat in a Grid, reading the parent field manually — the hierarchy is invisible.

The data layer and a generic UI component already exist; what's missing is the object-view integration that ties them together.

What already exists (foundation)

• tree field type (self-reference) — data layer supports { type: 'tree', reference: '<self>' }, i.e. a single-parent pointer. Unambiguous, unlike Airtable's many-to-many links (which is why Airtable has no tree view; we don't have that blocker). See packages/app-shell/src/views/metadata-admin/previews/field-types.ts:36.
• Generic TreeViewSchema component — expand/collapse tree renderer, but it consumes TreeNode[] and is not bound to object record data (more like a folder browser). packages/components/src/renderers/data-display/tree-view.tsx.

What's missing (the gap)

The 10 object view types (list / grid / kanban / calendar / timeline / map / gallery / gantt / chart / detail, see packages/types/src/views.ts:28) do not include tree. The view-type → component dispatch switch at packages/plugin-view/src/ObjectView.tsx:685 has no case 'tree', and there is no plugin-tree package.

Why not just use Grouping (à la Airtable)?

Grouping handles fixed-depth hierarchies (dept → team → person). It cannot express an arbitrary-depth self-referencing tree (A's parent is B, B's parent is C, …unbounded). That is precisely the business-unit / org-chart case and the reason we need a real tree view, not grouping.

Proposed scope

Start with tree-grid (a tree table — columns, with the first column indented/expandable), since it subsumes a plain tree.

1. Add 'tree' to the ViewType union — packages/types/src/views.ts.
2. New plugin-tree package (mirror plugin-kanban / plugin-gantt structure). Core work: transform flat records with a self-referencing parent field into a nested structure, feed into the existing TreeViewSchema / a tree-grid renderer.
3. Register case 'tree' in the dispatch switch — packages/plugin-view/src/ObjectView.tsx.
4. Add tree icon + label to ViewSwitcher (plugin-list).

Open questions

• Config surface: which field is the parent pointer (auto-detect the tree/self-reference field, or explicit tree.parentField?), root filter, default expand depth, lazy-load vs. fetch-all.
• Server-side: fetch the whole subtree at once vs. lazy children fetch per node (perf for large trees).
• Drag-to-reparent — in scope for v1 or a follow-up?

Acceptance (v1)

• A self-referencing object renders as an indented, expand/collapse tree-grid with columns.
• Parent field auto-detected from the object's tree field; configurable override.
• Reads via the generic data API (no bespoke endpoint).