fix: make plugin initialization idempotent for multiple imports

[dusan-maintains]

Summary

• add an initialization guard that returns early when jquery-modal is already attached
• prevent duplicate plugin setup when the script is imported multiple times
• keep the original modal instance state/event bindings intact

Why

Issue #309 reports breakage when jquery-modal is imported more than once. Re-initializing the plugin creates a fresh internal state that can desynchronize auto-close behavior from active modal instances.

Changes

• jquery.modal.js

Test Plan

• include jquery.modal.js twice and verify auto-close still works
• include once and verify normal behavior remains unchanged

Compatibility

• no API changes
• idempotent guard only affects duplicate-load scenario

Closes #309

[dusan-maintains]

Maintainer note: this addresses #309 by making initialization idempotent under duplicate imports. If there is a preferred guard condition/style, I can adjust quickly.