Bootstrap frontend unit tests with bun:test; cover token-refresh logic

Commit notes

Ten behavioral tests for eden.ts using Bun's built-in runner (no new dependencies): storage round-trip, single-flight refresh dedup (two concurrent refreshes -> one request), token preservation on network errors vs clearing + auth:expired on definitive rejection, SSE ticket preference with legacy-token fallback, and authFetch header injection. Adds the frontend "test" script that the CI frontend job runs.

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