Refactor index.html: copy updates, accessibility improvements, and JS event handling

[Limsym]

Motivation

• Improve user-facing copy and page metadata to be more concise and user-friendly.
• Enhance accessibility and semantics by adding proper label associations and ARIA attributes.
• Replace inline handlers with event listeners and harden external navigation with noopener,noreferrer for security.

Description

• Updated page title and meta description and simplified UI text such as subtitles, placeholders, and example hints.
• Added for attributes on labels, id on inputs, role="status" and aria-live="polite" on the message container, and type="button" on platform tag buttons for better accessibility and form behavior.
• Replaced inline onclick with event listeners attached in DOMContentLoaded, changed keypress to keydown for Enter handling, and wired platform tag clicks to selectPlatform(platformId, element) to avoid relying on event.target.
• Use window.open(targetUrl, '_blank', 'noopener,noreferrer') to open external links securely.

Testing

• No automated tests were executed for this change.

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Codex Task

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