test(unit): broaden coverage — quota formula + auth token/JWT decode
Backend: extract the per-user quota formula from compute_storage_quota into a
pure `quota_limit_bytes(free_disk, tolerance, active)` (behavior-preserving) and
unit-test it: tolerance scaling, floor of fractional results, active<1 clamped to
1 (divide-by-zero guard), zero free disk, single-uploader identity.
Frontend (jsdom + browser:true mock): auth.ts token storage and JWT claim decode
— setAuth/getToken/getPin round-trip, clearAuth keeps the PIN (for recovery),
clearPin, getExpiry (exp seconds→ms Date; null on missing/malformed), getRole
(role claim; null on absent/malformed). Adds jsdom devDependency.
Backend: 25 passing. Frontend: 18 passing.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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