test: establish shared UI migration gate

[uermel]

Summary

• add the first real pytest and offscreen-Qt gate for copick-shared-ui
• encode the metadata-blind thumbnail selection defect as the only expected failure
• add import, cache-isolation, and storage-selection behavioral coverage without AST parsing or structural source guards
• isolate every thumbnail cache in pytest-managed temporary storage
• run Python 3.11 through 3.14 in CI and align the Black policy used by the lock and pre-commit

Stack

This is stack layer 1 of 4 and targets the new v2.0 branch directly.

1. this PR — migration test gate
2. metadata-driven Zarr 3 runtime and thumbnail fix
3. backend, reconnect, artifact, and performance validation
4. v2.0 alpha release channel

Validation

• full layer-1 suite: 5 passed, 1 expected failure
• the cache-isolation regression proves worker construction stays under pytest's temporary directory
• Ruff and Black: passed
• compile/import smoke and package build: passed

This PR is part of #49, but does not close it. The broader fixture/smoke matrix remains tracked there.

Part of #49
Part of #53