refactor(session): extract useFileSession, fixing in-place state mutation

[killerwolf]

Summary

• App.tsx owned SessionState via useState, mutating arrays in place inside setState updaters (.push()/.pop() on the previous state's arrays after only a shallow copy) — a real correctness bug, not just style. FileViewer.tsx co-owned the "perform an action" flow with no single place owning what happens on delete.
• Extract src/hooks/useFileSession.ts: a 5-method interface (startSession/keep/deleteFile/undo/reset) owning the whole session, doing immutable state transitions throughout.
• deleteFile is the one async method — it awaits moveToTrash and only commits the "deleted" transition on success, so a failed trash operation can no longer be recorded as if it succeeded. Failure propagates as a rejection; FileViewer catches it and shows an alert instead of the previous fire-and-forget .catch(console.error).
• The hook exposes a derived isComplete flag; App.tsx reacts to it via an effect instead of re-deriving "did we finish" from raw currentIndex/files.length itself (a Feature Envy finding from review, fixed pre-commit).
• Updates CONTRIBUTING.md's stale test-coverage note.

Test plan

• npm run typecheck passes (all 3 configs)
• npm test — 50/50 pass (12 new: useFileSession.test.ts covers all 5 methods via renderHook, including the core fix — state is byte-for-byte unchanged when moveToTrash rejects — and the new isComplete derivation)
• 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); both Standards findings (a Feature Envy leak, duplicated undo branches) fixed pre-commit, Spec review clean
• Confirmed via git diff that CompletionScreen.tsx, playActionSound, the confirm dialog, and keyboard shortcuts are untouched
• Not verified interactively: this is an Electron renderer that depends on window.electronAPI (only present via Electron's preload bridge) — running it under plain Vite crashes immediately on an unrelated pre-existing component (UpdateNotifier), confirmed this isn't a regression by reproducing the same crash's cause. No tooling available here to drive a real Electron window, so this change is verified via the automated test suite only, not a manual click-through.

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