Extend automated test coverage

[killerwolf]

The project currently has zero automated tests — no test framework installed, no *.test.*/*.spec.* files anywhere in src/ or electron/.

Why Vitest

Fits naturally with the existing Vite build setup, fast, minimal config, good TypeScript + React support (via @testing-library/react).

Scope

• npm install -D vitest @testing-library/react @testing-library/jest-dom jsdom
• Add a test script and basic vitest.config.ts (reusing vite.config.ts aliases)
• Wire npm test into release.yml's test job so it's a real CI gate
• Initial coverage targets (highest value first):
  • electron/main.ts: getFileType() extension classification, scanFolder() filtering/sorting
  • Settings persistence round-trip (get/save via the IPC handlers)
  • FileViewer/CompletionScreen state transitions (delete/keep/undo)
• Not aiming for full coverage in the first pass — establish the harness and cover the riskiest logic (file classification, undo stack) first

[killerwolf]

Retitling: the suite is no longer Vitest. The app moved to Rust/GPUI in 2.0 (ADR 0002), and tests are now #[cfg(test)] modules run by cargo test. The goal of the issue is unchanged.

Covered today (38 tests):

• session.rs — keep/toss/undo, completion, progress, and that a toss which never reaches record_toss leaves no trace
• settings.rs — round-trip, defaults, malformed JSON, and reading settings.json written by the Electron build
• files.rs — extension classification, the "every supported extension has a kind" invariant, size formatting including the TB clamp that used to render 1 undefined, and folder scanning
• preview.rs — the Quick Look bridge against real files, plus text truncation and missing-file handling
• update.rs — version comparison, including 1.10.0 > 1.9.0 which a string compare gets wrong
• sound.rs — WAV structure and that the synthesis stays inside its headroom

Not covered:

• Anything in ui.rs or the QuickToss view — no rendering, key dispatch, or focus behaviour is exercised. This is the real gap. GPUI Kit ships a test-support feature for rendering components in headless windows and dispatching pointer/keyboard events, which is the obvious way in.
• update::check itself — the version comparison is tested, the HTTP call isn't.
• files::move_to_trash — one line over the trash crate, and a test would mostly assert that crate works.