refactor: dry improvements in web UI and OCPP mock server

refactor: dry improvements in web UI and OCPP mock server

Web UI:
- extract localStorage key constants (UI_SERVER_CONFIGURATION_INDEX_KEY,
  TOGGLE_BUTTON_KEY_PREFIX, SHARED_TOGGLE_BUTTON_KEY_PREFIX)
- add deleteLocalStorageByKeyPattern() utility with safe collect-then-
  delete iteration
- add getWebSocketStateName() utility replacing duplicated switch
- remove redundant .trim() calls (already handled by v-model.trim)

OCPP mock server:
- extract FALLBACK_TRANSACTION_ID constant and shared helper method
- replace 20-case match/case with _COMMAND_HANDLERS dict dispatch
- unify _parse_set/get_variable_specs into shared _parse_variable_specs
- add ClassVar annotation for proper mypy typing