chore: remove dead modules confirmed unused in-repo and across all store plugins

- src/common/cli.py: imports a 'ledmatrix_common' package that exists
  nowhere (not in this repo, any requirements file, or the plugin
  monorepo), so it cannot ever have run; its README section claimed
  scripts/dev/* used it, which was also untrue.
- src/web_interface/logging_config.py: zero callers — the web app uses
  web_interface/logging_config.py (a different module), and nothing
  imports the src copy.
- handle_errors decorator in src/web_interface/error_handler.py: zero
  call sites (the module's response helpers stay — they are used).
- ConfigManager.get_clock_config(): reads a 'clock' config key that no
  longer exists anywhere; only caller was its own unit test.

Deliberately kept despite zero in-repo callers: DisplayError,
src/common/config_helper.py and display_helper.py — all documented as
plugin-facing API (docs/PLUGIN_ERROR_HANDLING.md, src/common/README.md),
and third-party plugins outside the official monorepo cannot be
enumerated. Verified against a fresh clone of ledmatrix-plugins (43
plugins): zero references to any removed symbol.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SXb4mKcAkVaxkeTb3YnAdr

test/test_config_manager.py

@@ -383,19 +383,6 @@ class TestConfigHelpers:
         display_config = manager.get_display_config()
         assert display_config["hardware"]["rows"] == 32
     
-    def test_get_clock_config(self, tmp_path):
-        """Test getting clock config."""
-        config_file = tmp_path / "config.json"
-        config_data = {"clock": {"format": "12h"}}
-        
-        with open(config_file, 'w') as f:
-            json.dump(config_data, f)
-        
-        manager = ConfigManager(config_path=str(config_file))
-        manager.load_config()
-        
-        clock_config = manager.get_clock_config()
-        assert clock_config["format"] == "12h"
 
 
 class TestPluginConfigManagement: