docs: add CLAUDE.md for Claude Code guidance

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CLAUDE.md

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+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Commands
+
+| Command | Description |
+|---------|-------------|
+| `npm run dev` | Start Vite dev server + Electron |
+| `npm run build` | Full production build (native helpers → TypeScript → Vite → Electron Builder) |
+| `npm run build:win` | Windows build (includes WGC capture) |
+| `npm run build:mac` | macOS build |
+| `npm run build:linux` | Linux build |
+| `npm run lint` | Biome lint check |
+| `npm run lint:fix` | Biome auto-fix |
+| `npm run format` | Biome format |
+| `npm run test` | Run Vitest (single run) |
+| `npm run test:watch` | Vitest in watch mode |
+| `npm run i18n:check` | Validate i18n locale file structure against `en` |
+
+Run a single test file: `npx vitest run src/lib/exporter/gifExporter.test.ts`
+
+## Architecture
+
+**Electron + React + Vite** desktop screen recorder and video editor.
+
+### Process Model
+
+- **Main process** (`electron/main.ts`): Window lifecycle, tray icon, native recording, file I/O, permissions, auto-cleanup
+- **Preload** (`electron/preload.ts`): contextBridge API exposing ~40 IPC methods to renderer
+- **IPC handlers** (`electron/ipc/handlers.ts`): All main↔renderer communication; recording pipelines, file operations, project persistence, shortcuts, cursor telemetry
+- **Renderer** (`src/`): React SPA routed by `?windowType=` URL parameter (no router library)
+
+### Three Window Types
+
+1. **HUD Overlay** (`?windowType=hud-overlay`): 500×155 floating bar at screen bottom for recording controls. Transparent, always-on-top.
+2. **Editor** (`?windowType=editor`): Main video editor window (1200×800, maximized). Houses timeline, playback, annotations, export.
+3. **Source Selector** (`?windowType=source-selector`): 620×420 popup for picking capture sources.
+
+`App.tsx` switches components based on the `windowType` query param.
+
+### Recording Pipeline (Platform-Specific)
+
+- **macOS**: ScreenCaptureKit via compiled Swift helpers (`scripts/build-native-helpers.mjs`)
+- **Windows**: WGC (Windows.Graphics.Capture) via C++ CMake build (`scripts/build-wgc-capture.mjs`), fallback to FFmpeg
+- **Linux**: Chromium `getDisplayMedia`
+- Core recording logic: `src/hooks/useScreenRecorder.ts` (60 FPS target, adaptive bitrate)
+
+### Export Engine (`src/lib/exporter/`)
+
+GPU-accelerated pipeline: WebCodec streaming decode → PIXI.js frame rendering (zoom, crop, annotations, cursor) → MP4 mux (mp4box) or GIF (gif.js with Web Workers). Key files: `videoExporter.ts`, `gifExporter.ts`, `frameRenderer.ts`, `streamingDecoder.ts`, `muxer.ts`.
+
+### State Management
+
+No external state library. `VideoEditor.tsx` uses ~50 `useState` hooks. Undo/redo via `useRef` history stacks. Only two React Contexts: `I18nContext` and `ShortcutsContext`.
+
+### i18n System
+
+- Config: `src/i18n/config.ts` — languages: `en` (source), `es`
+- Locale files: `src/i18n/locales/{lang}/{namespace}.json`
+- 7 namespaces: `common`, `launch`, `editor`, `timeline`, `settings`, `dialogs`, `shortcuts`
+- Implementation: `src/contexts/I18nContext.tsx` — compile-time JSON import, recursive key lookup, `{{var}}` interpolation
+- Fallback chain: current language → English → provided fallback → raw key
+- Run `npm run i18n:check` after any locale changes
+- See `TRANSLATION_GUIDE.md` for contributor workflow
+
+### UI Layer
+
+- Base components: `src/components/ui/` — Radix UI primitives wrapped with shadcn/ui patterns
+- Styling: Tailwind CSS with CSS variable theming, class-based dark mode
+- Icons: lucide-react + react-icons
+- Timeline: `dnd-timeline` library for drag-and-drop editing
+- Path alias: `@/` → `src/`
+
+### Project File Format
+
+`.recordly` files (legacy `.openscreen` supported). Serialization in `src/components/video-editor/projectPersistence.ts`.
+
+## Code Quality
+
+- **Biome** is the primary linter/formatter (tab indent, LF, 100 char width)
+- **Strict TypeScript**: `noExplicitAny: error`, no unused variables/params
+- **Hook rules enforced**: `useHookAtTopLevel: error`
+- **Import organization**: automatic via Biome
+- Tests use Vitest + fast-check (property-based testing). Test files: `src/**/*.{test,spec}.{ts,tsx}`