Add all nodes and edges from the database to the canvas

> [!IMPORTANT]
> **Internal for now.** This feature is gated on a guardrail design decision (see the child spike). It is not open to community contribution until that decision lands and this notice is removed.

## Description

Add a way to load **all** nodes and edges from the connected graph database into the canvas at once, rather than expanding incrementally. Useful for small-to-moderate graphs where a user wants the whole picture immediately.

## The catch — must not run unguarded on a large database

Loading an entire large database into the canvas would overwhelm both the database and the browser. This feature **must not ship without a guardrail** that protects against accidental execution on a very large graph. The guardrail's mechanism and UX are an open design question, owned by the child spike below — this feature is **blocked by** it.

## Notes

- Relies on the batched detail-fetching shipped in #915 (closed) so bulk loads issue few requests, not one per element.

> [!IMPORTANT]
> Internal only — this issue is maintained by the core team and is not accepting external contributions.

Start with the child spike that owns the guardrail design, then review the batched detail-fetching from #915. The feature is done when all nodes and edges from a connected graph database can load into the canvas without shipping or running unguarded on a very large graph.

Written by the indexing model from the issue text.