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/vite.config.ts

@@ -14,6 +14,13 @@ export default defineConfig({
       reporter: ['text', 'html'],
       include: ['src/**/*.{ts,tsx}'],
       exclude: ['src/**/*.d.ts', 'src/test/**', 'src/main.tsx'],
+      // Baseline floors. Each refactor PR raises these; never lower.
+      thresholds: {
+        lines: 0,
+        functions: 0,
+        branches: 0,
+        statements: 0,
+      },
     },
   },
   server: {