Set up a frontend test runner (vitest)

[eipasteur]

The frontend has no test runner. PR #236 added the first frontend test (frontend/src/services/sprintGraph.test.ts, 8 cases for extractPrs), but:

• it broke the build job because tsc -b compiled it and vitest isn't a frontend dependency;
• as a stopgap, feat(review): surface all PRs in multi-repo review with per-PR tabs #236 excludes *.test.ts(x) from the app build (tsconfig.app.json), so the test is no longer compiled nor executed.

We need a real frontend test setup so that test (and future ones) actually run:

• add vitest as a frontend devDependency
• add frontend/vitest.config.ts that extends vite.config (needed because sprintGraph.ts imports ./api which uses import.meta.env)
• add a test script in frontend/package.json
• wire a frontend test step into CI
• re-enable / confirm sprintGraph.test.ts runs

Will be needed for the upcoming UI work (#184 / #185).