Add URL whitelist enforcement for Kiosk-mode and enhance configuration management

- Introduced `allowed_urls` in configuration to specify whitelisted domains/patterns.
- Implemented `RestrictedWebEngineView` to enforce URL restrictions in the web view.
- Updated `MainWindow` to utilize the new restricted web view and added navigation toolbar.
- Enhanced unit tests for configuration and restricted web view to cover new functionality.

src/webdrop_bridge/config.py

@@ -27,6 +27,7 @@ class Config:
         log_level: Logging level (DEBUG, INFO, WARNING, ERROR)
         log_file: Optional log file path
         allowed_roots: List of whitelisted root directories for file access
+        allowed_urls: List of whitelisted URL domains/patterns (empty = no restriction)
         webapp_url: URL to load in embedded web application
         window_width: Initial window width in pixels
         window_height: Initial window height in pixels
@@ -41,6 +42,7 @@ class Config:
     log_level: str
     log_file: Path | None
     allowed_roots: List[Path]
+    allowed_urls: List[str]
     webapp_url: str
     window_width: int
     window_height: int
@@ -71,6 +73,7 @@ class Config:
         log_level = os.getenv("LOG_LEVEL", "INFO").upper()
         log_file_str = os.getenv("LOG_FILE", "logs/webdrop_bridge.log")
         allowed_roots_str = os.getenv("ALLOWED_ROOTS", "Z:/,C:/Users/Public")
+        allowed_urls_str = os.getenv("ALLOWED_URLS", "")
         webapp_url = os.getenv("WEBAPP_URL", "file:///./webapp/index.html")
         window_width = int(os.getenv("WINDOW_WIDTH", "1024"))
         window_height = int(os.getenv("WINDOW_HEIGHT", "768"))
@@ -121,12 +124,19 @@ class Config:
         if not webapp_url:
             raise ConfigurationError("WEBAPP_URL cannot be empty")
 
+        # Parse allowed URLs (empty string = no restriction)
+        allowed_urls = [
+            url.strip() for url in allowed_urls_str.split(",")
+            if url.strip()
+        ] if allowed_urls_str else []
+
         return cls(
             app_name=app_name,
             app_version=app_version,
             log_level=log_level,
             log_file=log_file,
             allowed_roots=allowed_roots,
+            allowed_urls=allowed_urls,
             webapp_url=webapp_url,
             window_width=window_width,
             window_height=window_height,