test: add Vitest suite with 86% coverage threshold
- Add Vitest + jsdom test framework with V8 coverage
- 118 tests across 7 test files covering AppState, ProgressManager, EpisodeManager, FileListManager, fileUtils, main.js logic, renderer escapeHtml, preload module polyfill
- Fix async bug in extractFileDuration (await in non-async callback)
- CI enforces 85% coverage threshold on testable modules
- Exclude Electron IPC wrappers (FileManager, SearchManager, TagManager, ModalManager, UIManager) from coverage as they require Electron runtime
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