Drive the worker Auth domain to 100% coverage:
- AuthContext.tsx: 91.3/80 -> 100/100 (signIn body, mount-restore branches, useAuth guard)
- Login.tsx: 78/50 -> 100/100 (handleSubmit success/error/pending, submit disable)
- auth-storage.ts: already 100 (no change)
New Login.test.tsx adds the domain lifecycle test: from the unauthenticated app,
a successful login stores the token and flips the context to the authenticated
tabbed shell. Extended AuthContext.test.tsx covers signIn delegation, mount
restore (token present/absent), and the out-of-provider throw.
No bugs found; code was already correct. Note: the design checklist's
'clears on 401' worker behavior is not implemented (no global 401 interceptor)
and is left as-is (missing feature, not a defect).
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