Blabber footer alignment

[jasperf]

This pull request introduces several updates to the iwz-banner-container-plugin, focusing on improving banner alignment behavior, standardizing unique ID logic for banners, and enhancing code quality and user experience. The most notable changes include updates to banner rendering logic, CSS enhancements for desktop and mobile alignment, and modifications to the plugin version and constants.

Banner Rendering Logic Enhancements:

• Updated the display_blabber_footer_start_banner() method to initialize banner indexing at 1 instead of 0 for consistency across banner types. (F4be54ddL390R390)
• Modified the wrap_banner_html() method to include unique IDs for banners based on their index and location, improving targeting and alignment.

CSS Improvements:

• Added desktop-specific alignment styles (left, center, right) using flexbox and media queries, ensuring alignment settings are respected for screens 769px and above. [1] [2]
• Enhanced mobile-specific centering styles for screens 768px and below, maintaining compatibility and proper display.

Documentation Updates:

• Expanded the README.md to clarify desktop vs. mobile behavior and alignment settings, providing better guidance for plugin users.

Version and Constants:

• Updated the plugin version to 1.9.15 and corresponding constants to reflect the latest changes. [1] [2]

Changelog Additions:

• Documented all notable updates in CHANGELOG.md, including bug fixes, enhancements, and technical improvements for versions 1.9.13 through 1.9.15.