fix(ui-web): skip dev-only diagnostics under Vitest (#1897)

fix(ui-web): skip dev-only diagnostics under Vitest (#1897)

Extract isDev() helper in core/env.ts that returns true only in real
DEV (excludes production build and Vitest). Migrate the 7 existing
import.meta.env.DEV check sites to the helper.

Primary motivation: validateTokenContract schedules a
requestAnimationFrame whose body calls console.warn for missing CSS
tokens. jsdom does not resolve --color-* tokens, so every theme/skin
switch under Vitest 4 queued a console.warn that fired after
environment teardown — surfacing as EnvironmentTeardownError on
Linux/Node 22; passes on macOS/Node 24.

Side effect: silences console.debug noise from storage.ts,
providers.ts, ToggleButton.vue under Vitest. dev/build behaviour
unchanged.

Adds regression test in tests/unit/shared/tokens/contract.test.ts.