feat: Implement default welcome page for missing web application

- Added a professional HTML welcome page displayed when no web application is configured.
- Enhanced `_load_webapp()` method to support improved path resolution for both development and bundled modes.
- Updated error handling to show the welcome page instead of a bare error message when the webapp file is not found.
- Modified unit tests to verify the welcome page is displayed in error scenarios.

build: Complete Windows and macOS build scripts

- Created `build_windows.py` for building Windows executable and optional MSI installer using PyInstaller.
- Developed `build_macos.sh` for creating macOS application bundle and DMG image.
- Added logging and error handling to build scripts for better user feedback.

docs: Add build and icon requirements documentation

- Created `PHASE_3_BUILD_SUMMARY.md` detailing the build process, results, and next steps.
- Added `resources/icons/README.md` outlining icon requirements and creation guidelines.

chore: Sync remotes script for repository maintenance

- Introduced `sync_remotes.ps1` PowerShell script to fetch updates from origin and upstream remotes.

tests/unit/test_main_window.py

@@ -180,8 +180,8 @@ class TestMainWindowWebAppLoading:
 
         assert window.web_view is not None
 
-    def test_load_nonexistent_file_shows_error(self, qtbot, tmp_path):
-        """Test loading nonexistent file shows error HTML."""
+    def test_load_nonexistent_file_shows_welcome_page(self, qtbot, tmp_path):
+        """Test loading nonexistent file shows welcome page HTML."""
         config = Config(
             app_name="Test",
             app_version="1.0.0",
@@ -205,10 +205,10 @@ class TestMainWindowWebAppLoading:
                 window._load_webapp()
                 mock_set_html.assert_called_once()
 
-                # Verify error message
+                # Verify welcome page is shown instead of error
                 call_args = mock_set_html.call_args[0][0]
-                assert "Error" in call_args
-                assert "not found" in call_args
+                assert "WebDrop Bridge" in call_args
+                assert "Application Ready" in call_args
 
 
 class TestMainWindowDragIntegration: