docs(config): update README with JSON configuration format

Update documentation to reflect the actual JSON-based configuration
system instead of the old INI-style format.

Changes:
- Updated Quick Start section to reference config.example.json
- Rewrote Configuration section with JSON examples
- Added detailed environment variable override documentation
- Included CI/CD usage examples
- Documented all supported environment variables

All acceptance criteria from Issue #6 verified:
✓ config.example.json created and documented
✓ load-config.sh correctly parses JSON
✓ Environment variables take precedence over JSON
✓ Validation script catches missing required fields
✓ Documentation clear for new developers
✓ CI can inject credentials via env vars

Related: #6

README.md

@@ -113,19 +113,26 @@ Inside the container, all devkitARM tools are available in PATH. Run `make` to b
 For testing with real Nextcloud servers, copy the example configuration:
 
 ```bash
-cp config.example config
+cp config.example.json config.json
 ```
 
-Edit `config` and add your test Nextcloud credentials:
+Edit `config.json` and add your test Nextcloud credentials:
 
-```ini
-# Local test configuration - DO NOT COMMIT THIS FILE
-NEXTCLOUD_URL=https://your-nextcloud-instance.com
-NEXTCLOUD_USER=your-username
-NEXTCLOUD_PASSWORD=your-password
+```json
+{
+  "nextcloud": {
+    "url": "https://your-nextcloud-instance.com",
+    "username": "your-username",
+    "password": "your-app-password"
+  },
+  "settings": {
+    "maxRecentFolders": 5,
+    "uploadChunkSize": 10485760
+  }
+}
 ```
 
-**Note**: The `config` file is in `.gitignore` and will never be committed.
+**Note**: The `config.json` file is in `.gitignore` and will never be committed.
 
 ## 🔧 Building
 
@@ -161,31 +168,71 @@ Platform-specific build instructions:
 
 ### Local Development Configuration
 
-The `config` file (copied from `config.example`) is used for local testing:
+The project uses a JSON configuration file for managing test credentials and build options.
 
-```ini
-NEXTCLOUD_URL=https://cloud.example.com
-NEXTCLOUD_USER=testuser
-NEXTCLOUD_PASSWORD=testpass
-MAX_RECENT_FOLDERS=5  # Build-time option
+**Setup:**
+```bash
+# Copy the example configuration
+cp config.example.json config.json
+
+# Edit config.json with your credentials
+nano config.json
 ```
 
-### CI/CD Environment Variables
-
-For CI/CD pipelines, use environment variables to override the local config:
-
-- `NEXTCLOUD_TEST_URL` - Test server URL
-- `NEXTCLOUD_TEST_USER` - Test username  
-- `NEXTCLOUD_TEST_PASSWORD` - Test password
-- `MAX_RECENT_FOLDERS` - Number of recent folders to track
-
-Example:
+**Configuration structure:**
+```json
+{
+  "nextcloud": {
+    "url": "https://cloud.example.com",
+    "username": "testuser",
+    "password": "your-app-password"
+  },
+  "settings": {
+    "maxRecentFolders": 5,
+    "uploadChunkSize": 10485760
+  }
+}
+```
 
+**Loading configuration:**
 ```bash
-NEXTCLOUD_TEST_URL=https://cloud.tomusan.com \
-NEXTCLOUD_TEST_USER=ci-user \
-NEXTCLOUD_TEST_PASSWORD=ci-pass \
-podman run --rm -v ./:/project:z nextcloud-share-3ds make
+# Load configuration and export as environment variables
+source scripts/load-config.sh
+
+# Validate configuration
+./scripts/validate-config.sh
+```
+
+### Environment Variable Overrides
+
+Environment variables take precedence over JSON configuration values. This is useful for CI/CD pipelines where you don't want to store credentials in files.
+
+**Supported environment variables:**
+- `NEXTCLOUD_URL` - Nextcloud server URL (required)
+- `NEXTCLOUD_USER` - Username for authentication (required)
+- `NEXTCLOUD_PASSWORD` - Password or app password (required)
+- `MAX_RECENT_FOLDERS` - Number of recent folders to track (default: 5)
+- `UPLOAD_CHUNK_SIZE` - Upload chunk size in bytes (default: 10485760)
+- `CONFIG_FILE` - Path to config file (default: config.json)
+
+**CI/CD Example:**
+```bash
+# Override configuration with environment variables
+export NEXTCLOUD_URL=https://cloud.tomusan.com
+export NEXTCLOUD_USER=ci-user
+export NEXTCLOUD_PASSWORD=ci-pass
+
+# Load any remaining config from file
+source scripts/load-config.sh
+
+# Run tests
+podman run --rm \
+  -e NEXTCLOUD_URL \
+  -e NEXTCLOUD_USER \
+  -e NEXTCLOUD_PASSWORD \
+  -v ./:/project:z \
+  tomusan/devkitarm-3ds:latest \
+  make test
 ```
 
 ## 🧪 Testing