Add unit tests for existing selectors and reducers in wizardSlice.ts to create a safety net before structural changes. This covers the 72 selectors and reducers with non-trivial logic (filesystem, users, content).

Acceptance Criteria:

• Unit tests cover all selectors in wizardSlice.ts, validated against RootState
• Unit tests cover filesystem reducers (disk management, partitions, FSC mode)
• Unit tests cover users reducers (user management, user groups)
• Unit tests cover content reducers (repositories, packages, modules, groups, snapshotting)
• Tests focus on behavior, not implementation details
• All tests pass

Notes:

• Generated/vibecoded tests as a starting point are acceptable
• We won't co-locate the tests with the code until after some of the reorganisation