Add regression coverage for the deprecated static mock flag in config

## Problem
web/src/config.ts exports USE_MOCK for backward compatibility, but the existing test only covers API_BASE_URL. The deprecated flag is not protected by a regression test, so it could be accidentally flipped and reintroduce the legacy mock-data code path.

## Expected behavior
- USE_MOCK remains alse for the current Studio runtime.
- A focused Vitest case fails if the flag is changed.

## Scope
Only add regression coverage for web/src/config.test.ts. No runtime code changes.

## Acceptance criteria
- [ ] A Vitest assertion verifies USE_MOCK === false.
- [ ] The targeted test file passes.

Start with web/src/config.ts to confirm the exported USE_MOCK value, then open web/src/config.test.ts and run that targeted Vitest file. Add the focused assertion described in the issue; done means the test verifies USE_MOCK === false and the targeted test passes.

Written by the indexing model from the issue text.