# Implementation Summary: File Listing Feature

## Problem
The frontend was unable to list existing files because the backend API did not have an endpoint to retrieve the list of files in the data directory.

## Root Cause
The backend API handler only supported individual file operations (`GET /api/{filename}.md`, `POST /api/{filename}.md`, etc.) but had no endpoint to list all files in the data directory. The frontend was attempting to fetch `/api` to get the file list, but this route was not configured.

## Solution
Added a new endpoint `GET /api` that returns a JSON list of all markdown files in the data directory.

### Changes Made

#### 1. Backend API (`backend/internal/api/api.go`)
- Added `handleListFiles()` method that:
  - Reads all files from the data directory
  - Filters to only include `.md` files
  - Returns JSON response with format: `{"files": ["file1.md", "file2.md", ...]}`
- Modified `handleGet()` to check if filename is empty and call `handleListFiles()` if so

#### 2. Backend Server (`backend/internal/server/server.go`)
- Added route handler for `/api` with GET method
- Kept existing route handler for `/api/{filename:.+.md}`

### API Endpoints

#### New Endpoint
- **Method:** GET
- **Path:** `/api`
- **Response:** `{"files": ["file1.md", "file2.md", ...]}`
- **Status Codes:**
  - 200 OK - Success
  - 500 Internal Server Error - Failed to read directory

#### Existing Endpoints (Unchanged)
- **GET** `/api/{filename}.md` - Get file content
- **POST** `/api/{filename}.md` - Create file
- **PUT** `/api/{filename}.md` - Update file
- **DELETE** `/api/{filename}.md` - Delete file

### Testing
Added comprehensive tests in `backend/tests/file_listing_test.go`:
- `TestFileListing` - Verifies multiple markdown files are listed
- `TestFileListingWithNonMarkdownFiles` - Verifies only `.md` files are returned
- `TestFileListingEmptyDirectory` - Verifies empty array for empty directory

All existing tests continue to pass.

### Frontend Compatibility
The frontend (`frontend/src/App.tsx`) already expects the correct response format:
```typescript
const loadFiles = async () => {
  const response = await fetch('/api')
  if (response.ok) {
    const data = await response.json()
    setFiles(data.files || [])  // Expects { files: string[] }
  }
}
```

No frontend changes were required.

## Verification
The implementation was verified by:
1. Running all existing tests - ✅ PASS
2. Running new file listing tests - ✅ PASS
3. Manual API testing with curl - ✅ WORKING
4. Frontend build verification - ✅ SUCCESS

## Files Modified
- `backend/internal/api/api.go` - Added file listing functionality
- `backend/internal/server/server.go` - Added `/api` route
- `backend/tests/file_listing_test.go` - Added comprehensive tests (NEW FILE)