Improve coverage for core analysis flows

[lemonmindyes]

Summary

• Expand backend tests for DeepSeek request/error handling, PDF parsing, prompt building, settings normalization, export safety, code blueprint validation, and paper analysis fallback flows.
• Expand renderer tests for analysis status/error/download flows, resizable layout behavior, OutputPanel markdown/token/error states, PDF upload interactions, SettingsPanel behavior, and i18n fallback handling.
• Enforce 95% global coverage thresholds while excluding Electron/React entry glue files from coverage scope.

Validation

• npm run test:coverage
• npm test
• npm run build