test(decnet_web): MSW-based test foundation for UI refactor

Phase 0 of the decnet_web refactor: stand up an MSW server, fixtures,
and a router-aware render helper so the upcoming god-component splits
(AttackerDetail first) can land with same-commit test coverage.

- msw devDep + setupServer wired into src/test/setup.ts
- src/test/server.ts re-exports server, http, HttpResponse, apiUrl()
- src/test/fixtures/{attacker,decky,canary,topology}.ts factories
- src/test/renderWithRouter.tsx wraps MemoryRouter + ToastProvider
- baseline coverage thresholds (0%) in vite.config.ts; raise per PR
- coverage/ added to decnet_web/.gitignore

Existing Orchestrator/AttackerDetail/ThemeLab tests stay on vi.mock
and continue to pass; new tests use MSW.

decnet_web/package.json

@@ -36,6 +36,7 @@
     "eslint-plugin-react-refresh": "^0.5.2",
     "globals": "^17.4.0",
     "jsdom": "^29.1.1",
+    "msw": "^2.14.5",
     "typescript": "~6.0.2",
     "typescript-eslint": "^8.58.0",
     "vite": "^8.0.4",