refactor(electron): split main.ts into deep FileOperations/SettingsStore/Updater modules

[killerwolf]

Summary

• main.ts's QuickTossApp mixed five concerns — window lifecycle, IPC handler logic, folder-scan logic, settings persistence, and auto-updater orchestration — none of it unit-testable without a real Electron process
• Extract three deep modules:
  • electron/file-operations.ts — scanFolder/moveToTrash/getFileStats/fileExists/readFileAsBuffer as plain, window-free async functions, with a shared logAndRethrow helper codifying the error-handling policy (rethrow when there's no safe fallback, return a default when there is)
  • electron/settings-store.ts — a createSettingsStore(settingsPath) factory, taking the path as a parameter instead of computing it internally, so it's testable against a temp directory instead of the real userData folder
  • electron/updater.ts — auto-updater orchestration, its file logger, and openReleasePage, all window-free (takes an onStatus callback instead of touching BrowserWindow/webContents directly)
• main.ts keeps only window lifecycle and selectFolder (the one handler needing mainWindow), wiring the extracted modules' functions directly into the IPC handlers object
• Collapses a duplicated DEFAULT_SETTINGS object (previously defined twice, verbatim) into one shared constant
• Updates CONTRIBUTING.md's project layout and test-coverage notes to match

Test plan

• npm run typecheck passes (all 3 configs)
• npm test — 39/39 tests pass (14 new: file-operations.test.ts covers scanFolder/getFileStats/fileExists/readFileAsBuffer against real temp directories, no fs mocking; settings-store.test.ts covers the round-trip and corrupted-file fallback)
• npm run build — Vite + Electron compile cleanly
• npx biome check clean on changed files
• Reviewed against CONTRIBUTING.md standards and the original design (two-axis review); moveToTrash and updater.ts deliberately left untested — shell.trashItem and electron-updater's real update flow don't behave meaningfully outside a real Electron main process

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