Expand test coverage for analysis and layout features

[lemonmindyes]

Summary

• Add backend tests for DeepSeek streaming usage, API error handling, cancellation, and analysis result flow
• Add renderer tests for analysis status/timer/token UI, PDF panel behavior, i18n keys, and resizable layout persistence
• Add Vitest jsdom setup, Testing Library dependencies, and shared renderer utility tests

Tests

• npm test
• npm run build