refactor: comprehensive project improvements - Security: Enable CSP, add prompt boundary markers - Testing: Add vitest with 83 tests, Biome linter - Architecture: Extract App.tsx into focused hooks (1423→821 lines), split useAppView.ts into 4 files, split chat.rs into 5 submodules - Rust: Replace string enums with SessionStatus/AutonomyMode/MessageRole, deduplicate approval gate, batch disk writes, reduce cloning - Performance: React.lazy code splitting, React.memo on screens, optimize terminal buffer - Accessibility: aria-labels on all interactive controls - Config: private:true, explicit tailwindcss, meta description - Docs: Update SPEC.md and AGENTS.md#2