refactor(app): extract file session hook

[SulimanAbdulrazzaq]

Closes #92.

What changed

• move the active file state and tab snapshots into a typed useFileSession hook
• move open, restore, switch, new, save, save-as, close, reopen, reorder, session persistence, autosave, and external-change behavior behind that boundary
• keep UI-only state and link/navigation concerns in App
• preserve the existing AI-review autosave guard when the lifecycle code moves
• add hook-level coverage for opening, switching with dirty edits, clean close, and dirty-close confirmation

App.tsx is reduced from 2,248 to 1,513 lines; the extracted lifecycle is isolated in src/hooks/useFileSession.ts.

Validation

• npx bun install --frozen-lockfile
• npx bun run build — passed (tsc + Vite)
• npx bun x vitest run src/hooks/useFileSession.test.ts — 4 passed
• npx bun x vitest run --maxWorkers=1 — 41 files, 419 tests passed

The default parallel test command initially reached 418/419 with one unrelated DOCX export timeout; that file passed 13/13 when rerun, and the complete serialized suite then passed 419/419.