Add quick test runner and enhance run_tests script

- Introduced `quick_test.py` for running specific test categories (unit, integration, theme, all).
- Updated `run_tests.py` to improve test execution and reporting, including coverage.
- Removed outdated test scripts for keyboard shortcuts, menu theming, note saving, and entry updating.
- Added new test script `test_theme_changing.py` to verify theme changing functionality.
- Consolidated integration tests into `test_integration.py` for comprehensive testing of TheChart application.
- Updated theme manager to ensure color retrieval works correctly.
- Modified test constants to import from the correct module path.

src/theme_manager.py

@@ -298,18 +298,18 @@ class ThemeManager:
             }
 
         try:
-            # Get colors from current theme
-            bg = self.style.lookup("TFrame", "background") or "#ffffff"
-            fg = self.style.lookup("TLabel", "foreground") or "#000000"
+            # Get colors from current theme and convert to strings
+            bg = str(self.style.lookup("TFrame", "background") or "#ffffff")
+            fg = str(self.style.lookup("TLabel", "foreground") or "#000000")
 
             # Try to get better selection colors from different widget states
-            select_bg = (
+            select_bg = str(
                 self.style.lookup("TButton", "background", ["pressed"])
                 or self.style.lookup("TButton", "background", ["active"])
                 or self.style.lookup("Treeview", "selectbackground")
                 or "#0078d4"  # Modern blue fallback
             )
-            select_fg = (
+            select_fg = str(
                 self.style.lookup("TButton", "foreground", ["pressed"])
                 or self.style.lookup("TButton", "foreground", ["active"])
                 or self.style.lookup("Treeview", "selectforeground")