Add an integration_test harness (fake Cosmos + bypassed sign-in) for end-to-end flows

[yvanvds]

Context

The repo has no integration_test/ directory and no integration_test dev dependency. Every user-visible change is currently verified at the widget level only (e.g. test/features/lesson_content/lesson_content_page_test.dart mounts the real page over real services backed by test/helpers/in_memory_cosmos.dart), because booting the real app requires an Entra sign-in and a live Cosmos endpoint.

Found while working #4 (orphaned lesson content reassign): the start-work test doctrine asks for an end-to-end test on user-visible changes, and there is no harness to write one against.

Proposal

• Add integration_test/ with a Windows-desktop driver (flutter test integration_test/... -d windows).
• Provide a boot path that bypasses Entra (inject a fixed AccountIdentity) and swaps CosmosPaths for an in-memory container (reuse/extend InMemoryCosmos).
• Port the Content editor: reassign orphaned content items after goal re-import #4 reassign flow as the first e2e test: Doelen import (Replace) -> Lesinhoud shows orphan -> reassign -> subgoal shows content.
• Wire it into CI (build.yml) behind a separate job so the unit suite stays fast.

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