test(ui-kit): add a package-local vitest suite for @loopover/ui-kit's logic-bearing exports (#7495)

packages/loopover-ui-kit had no test script, vitest config, or test files, so its own
logic (relativeTimeFromNow, useIsMobile, the state-primitive copy branches) was only
verified incidentally through consuming apps. Adds a package-local vitest.config.ts +
setup (mirroring apps/loopover-miner-ui: jsdom + React plugin + testing-library
cleanup), a test script, the test devDependencies pinned to miner-ui's exact versions
(0 new lockfile packages), a tsconfig exclude so the dist build never ships a test
file, and colocated unit tests for utils, use-mobile, and state-views -- excluding the
separately-tracked onFailureNotify effect's call-count behavior. Not wired into
test:ci (out of scope per the issue); runs via npm --workspace @loopover/ui-kit test.

Closes #7437

Author: xfodev