fix(ui): preserve structured API error messages

[wunitb]

Summary

• extract user-facing messages from nested API error payloads instead of coercing objects to [object Object]
• apply the shared parser to session and control-plane UI requests
• keep unknown structured payloads fail-closed behind existing status-based fallback messages

Verification

• npm test -- --run — 132 files, 689 tests passed
• npm run build
• npx eslint src/services/errorMessage.ts src/services/sessionsApi.ts src/services/api.ts src/test/services/errorMessage.test.ts src/test/services/sessionsApi.test.ts
• git diff --check

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[wunitb]

Closing: this change was opened against the wrong upstream repository after I misattributed the downstream m5bp UI to the requested project scope.