test: set IS_REACT_ACT_ENVIRONMENT once per package in a vitest setup file

[librowski]

Summary

IS_REACT_ACT_ENVIRONMENT was set at the top of seven specs (six in ui, one in AI Studio), with two competing typings: a globalThis cast in five files and a declare global block with an eslint-disable no-var in the other two. Neither package has testing-library, so nothing set the flag centrally. In AI Studio the app tsconfig excludes spec files, so its declare global was never type-checked at all.

Changes

Per package (packages/ui, apps/ai-studio):

• vitest.setup.ts sets the flag once; the vitest test block loads it through setupFiles.
• types.d.ts declares var IS_REACT_ACT_ENVIRONMENT: boolean in declare global, so specs need no cast.
• The specs drop their local copies.
• ui only: tsconfig.json includes the setup file so tsc checks it (AI Studio picks it up by default, its tsconfig has no include), and vite-plugin-dts excludes it so nothing lands in dist.

Checks

• vitest ui 74 and AI Studio 16 passing. Negative control with setupFiles removed: the ui menu spec logs 44 "not configured to support act" warnings and AI Studio logs 8; with the setup none.
• A deliberate = 'x' in each setup file fails the package typecheck, so both files are type-checked.
• ui dist has no new files, publint and check:built-css pass; AI Studio builds.
• eslint and prettier clean on the touched files.