fix: correct blob mock so mikeApi tests pass on Node 22 and 24

From the PR description

Summary

Fixes 4 tests in frontend/src/app/lib/mikeApi.test.ts that fail locally on Node 24 with TypeError: blob.text is not a function, while passing in CI (Node 22).

Root cause

The tests already construct real Response objects (new Response(...)), so the request/response mocking itself was not the problem. The actual cause is in the shared Vitest jsdom test environment setup:

• Vitest's built-in jsdom environment always overrides globalThis.Blob with jsdom's own Blob implementation, which has never implemented .text(), .arrayBuffer(), or .stream() - only slice, size, and type.
• Node's built-in Response.prototype.blob() resolves its Blob constructor dynamically from globalThis at call time. On Node 24 this means response.blob() returns a jsdom-shaped blob missing those methods; on Node 22 it apparently does not exhibit this behavior, so the same tests pass there.
• Since the repo's own package.json pins engines.node: ">=22" with no upper bound, Node 24 is a legitimately supported local dev version, so this was a real (if narrow) bug, not just an unsupported-toolchain situation.

Fix

Restore the real Node Blob (imported directly from node:buffer, independent of the globalThis override jsdom performs) in frontend/vitest.setup.ts, which runs before every test file. This makes response.blob() behave identically on Node 22 and Node 24, and does not touch any production code path.

Changes

• frontend/vitest.setup.ts: restore globalThis.Blob to the native node:buffer Blob after jsdom's environment setup overrides it, with a comment explaining why.

Test plan

• cd frontend && npx vitest run src/app/lib/mikeApi.test.ts - 178/178 pass (previously 174/178, with the 4 tests named in #361 failing).
• cd frontend && npx vitest run (full frontend suite) - 98 files / 641 tests, all pass - confirming no regression from the global Blob change on other tests that rely on File/Blob/FormData (upload flows, clipboard export, etc).
• cd frontend && npx tsc --noEmit - clean.
• cd frontend && npx eslint vitest.setup.ts - clean.
• npm run build --prefix frontend was not run in this environment: this worktree's node_modules is a symlink to a sibling worktree (to avoid an npm install under a non-pinned local Node version per repo policy), and Turbopack refuses to build through a symlink that points outside its detected project root. Confirmed this Turbopack error is pre-existing and unrelated to this change by reproducing it identically with the fix stashed out. tsc --noEmit covers the type-checking this build step would otherwise gate.

Closes #361