feat: add CLAUDE.md for project guidance and architecture overview

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.
+
+## Project Overview
+
+EMLy is a Windows desktop email viewer for `.eml` and `.msg` files built with Wails v2 (Go backend + SvelteKit/Svelte 5 frontend). It supports viewing email content, attachments, and Italian PEC (Posta Elettronica Certificata) certified emails.
+
+## Build Commands
+
+```bash
+# Development mode with hot-reload
+wails dev
+
+# Production build (outputs to build/bin/EMLy.exe)
+wails build
+
+# Build for specific platform
+wails build -platform windows/amd64
+
+# Frontend type checking
+cd frontend && bun run check
+
+# Go tests
+go test ./...
+
+# Install frontend dependencies (also runs automatically on build)
+cd frontend && bun install
+```
+
+## Architecture
+
+### Wails Framework
+- Go backend methods are automatically exposed to frontend via generated TypeScript bindings
+- Bindings are generated in `frontend/src/lib/wailsjs/go/main/App.ts`
+- All App methods with exported names become callable from TypeScript
+- Wails runtime provides window control, dialogs, and event system
+
+### Backend Structure (Go)
+
+The `App` struct is the main controller, split across files by domain:
+
+| File | Responsibility |
+|------|----------------|
+| `app.go` | Core struct, lifecycle (startup/shutdown), configuration |
+| `app_mail.go` | Email reading: ReadEML, ReadMSG, ReadPEC, ShowOpenFileDialog |
+| `app_viewer.go` | Viewer windows: OpenImageWindow, OpenPDFWindow, OpenEMLWindow |
+| `app_screenshot.go` | Window capture using Windows GDI APIs |
+| `app_bugreport.go` | Bug report creation with screenshots and system info |
+| `app_settings.go` | Settings import/export to JSON |
+| `app_system.go` | Windows registry, encoding conversion, file explorer |
+
+Email parsing lives in `backend/utils/mail/`:
+- `eml_reader.go` - Standard EML parsing
+- `msg_reader.go` - Microsoft MSG (CFB format) parsing
+- `mailparser.go` - MIME multipart handling
+
+### Frontend Structure (SvelteKit + Svelte 5)
+
+**Routes** (file-based routing):
+- `(app)/` - Main app with sidebar layout, titlebar, footer
+- `(app)/settings/` - Settings page
+- `image/` - Standalone image viewer (launched with `--view-image=<path>`)
+- `pdf/` - Standalone PDF viewer (launched with `--view-pdf=<path>`)
+
+**Key patterns**:
+- Svelte 5 runes: `$state`, `$effect`, `$derived`, `$props` (NOT legacy stores in components)
+- State classes in `stores/*.svelte.ts` using `$state` for reactive properties
+- Traditional Svelte stores in `stores/app.ts` for simple global state
+- shadcn-svelte components in `lib/components/ui/`
+
+**Mail utilities** (`lib/utils/mail/`):
+- Modular utilities for email loading, attachment handling, data conversion
+- Barrel exported from `index.ts`
+
+### Multi-Window Support
+
+The app spawns separate viewer processes:
+- Main app uses single-instance lock with ID `emly-app-lock`
+- Image/PDF viewers get unique IDs per file, allowing multiple concurrent viewers
+- CLI args: `--view-image=<path>` or `--view-pdf=<path>` trigger viewer mode
+
+### Internationalization
+
+Uses ParaglideJS with compile-time type-safe translations:
+- Translation files: `frontend/messages/en.json`, `it.json`
+- Import: `import * as m from "$lib/paraglide/messages"`
+- Usage: `{m.mail_open_btn_text()}`
+
+## Key Patterns
+
+### Calling Go from Frontend
+```typescript
+import { ReadEML, ShowOpenFileDialog } from "$lib/wailsjs/go/main/App";
+const filePath = await ShowOpenFileDialog();
+const email = await ReadEML(filePath);
+```
+
+### Event Communication
+```typescript
+import { EventsOn } from "$lib/wailsjs/runtime/runtime";
+EventsOn("launchArgs", (args: string[]) => { /* handle file from second instance */ });
+```
+
+### Email Body Rendering
+Email bodies render in sandboxed iframes with links disabled for security.
+
+## Important Conventions
+
+- Windows-only: Uses Windows APIs (registry, GDI, user32.dll)
+- Bun package manager for frontend (not npm/yarn)
+- Frontend assets embedded in binary via `//go:embed all:frontend/build`
+- Custom frameless window with manual titlebar implementation