test: stub user-preference fetches in global setup to stop a cross-test ECONNREFUSED leak

[DavidBabinec]

Follow-up to #58/#59. Final root cause of the canvas-test flake that was failing the release Verify step.

Root cause

Admin surfaces load user preferences on mount — e.g. module-inserter favourites via useModuleInserterPreference → getUserPreference → a real fetch to /admin/api/cms/me/preferences/<key>. In tests that hits localhost and rejects with ECONNREFUSED.

The preference store is a module-level singleton whose load promise can outlive the component that triggered it. So the rejection isn't caught by the originating test — it surfaces during a later test's cleanup() (when React unmounts the leftover tree inside act), bundled as an AggregateError. That's why the failure landed on canvasFrameMounting's first test even though that test doesn't fetch anything.

It was masked while canvas frames mounted slowly (the breakpoint tests timed out before the preference fetch settled) and reappeared once #58 made frames mount synchronously — a pure render-timing change exposing a pre-existing leak.

Fix

One global stub in the test preload (src/__tests__/setup.ts): return the server's "never set" envelope ({ value: null }) for preference GETs, so the load resolves cleanly to each caller's own default with no network call. Suite-wide, there's no rejection left to leak.

• Tests that need specific preference data still override globalThis.fetch in their own setup (verified: the dedicated module-inserter preference tests pass).
• getUserPreference callers that inject a fetchImpl bypass this entirely.

Verification

• Proven the stub intercepts: an unmocked preference GET now returns 200 {value:null} and getUserPreference('module-inserter') resolves to null.
• Dedicated preference + canvas tests pass.
• Full suite: bun test 5432 pass / 0 fail; bun run build + bun run lint clean.

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