Replace global.fetch mocks with MSW in component tests

[grmbyrn]

Title: Replace global.fetch mocks with MSW in component tests

Description:
Many tests set global.fetch directly; MSW provides more accurate and isolated request mocking and better test hygiene.

Acceptance criteria:

• Add MSW dev/test setup and update jest.setup.ts to start/stop MSW.
• Replace global.fetch mocks in __tests__/* with MSW handlers.
• Ensure tests run reliably and restore network behavior between tests.

Labels: testing
Priority: medium