Add project documentation and translation guide (#174)

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
+
+HabitTrove is a gamified habit tracking PWA built with Next.js 15, TypeScript, and Jotai state management. Users earn coins for completing habits and can redeem them for rewards. Features multi-user support with admin capabilities and shared ownership of habits/wishlist items.
+
+## Essential Commands
+
+### Development
+- `npm run dev` - Start development server with Turbopack
+- `npm run setup:dev` - Full setup: installs bun, dependencies, runs typecheck and lint
+- `npm install --force` - Install dependencies (force flag required)
+
+### Quality Assurance (Run these before committing)
+- `npm run typecheck` - TypeScript type checking (required)
+- `npm run lint` - ESLint code linting (required)
+- `npm test` - Run tests with Bun
+- `npm run build` - Build production version
+
+### Docker Deployment
+- `npm run docker-build` - Build Docker image locally
+- `docker compose up -d` - Run with docker-compose (recommended)
+- Requires `AUTH_SECRET` environment variable: `openssl rand -base64 32`
+
+## Architecture Overview
+
+### State Management (Jotai)
+- **Central atoms**: `habitsAtom`, `coinsAtom`, `wishlistAtom`, `usersAtom` in `lib/atoms.ts`
+- **Derived atoms**: Computed values like `dailyHabitsAtom`, `coinsBalanceAtom`
+- **Business logic hooks**: `useHabits`, `useCoins`, `useWishlist` in `/hooks`
+
+### Data Models & Ownership
+- **Individual ownership**: `CoinTransaction` has single `userId`
+- **Shared ownership**: `Habit` and `WishlistItemType` have `userIds: string[]` array
+- **Admin features**: Admin users can view/manage any user's data via dropdown selectors
+- **Data persistence**: JSON files in `/data` directory with automatic `/backups`
+
+### Key Components Structure
+- **Feature components**: `HabitList`, `CoinsManager`, `WishlistManager` - main page components
+- **Modal components**: `AddEditHabitModal`, `AddEditWishlistItemModal`, `UserSelectModal`
+- **UI components**: `/components/ui` - shadcn/ui based components
+
+### Authentication & Users
+- NextAuth.js v5 with multi-user support
+- User permissions: regular users vs admin users
+- Admin dropdown patterns: Similar implementation across Habits/Wishlist pages (reference CoinsManager for pattern)
+
+### Internationalization
+- `next-intl` with messages in `/messages/*.json`
+- Supported languages: English, Spanish, German, French, Russian, Chinese, Japanese
+
+## Code Patterns
+
+### Component Structure
+```typescript
+// Standard component pattern:
+export default function ComponentName() {
+  const [data, setData] = useAtom(dataAtom)
+  const { businessLogicFunction } = useCustomHook()
+  // Component logic
+}
+```
+
+### Hook Patterns
+- Custom hooks accept options: `useHabits({ selectedUser?: string })`
+- Return destructured functions and computed values
+- Handle both individual and shared ownership models
+
+### Shared Ownership Pattern
+```typescript
+// Filtering for shared ownership:
+const userItems = allItems.filter(item => 
+  item.userIds && item.userIds.includes(targetUserId)
+)
+```
+
+### Admin Dropdown Pattern
+Reference `CoinsManager.tsx:107-119` for admin user selection implementation. Similar pattern should be applied to Habits and Wishlist pages.
+
+## Data Safety
+- Always backup `/data` before major changes
+- Test with existing data files to prevent data loss
+- Validate user permissions for all data operations
+- Handle migration scripts carefully (see PLAN.md for shared ownership migration)
+
+## Performance Considerations
+- State updates use immutable patterns
+- Large dataset filtering happens at hook level
+- Derived atoms prevent unnecessary re-renders