Add WCAG accessibility improvements

[akshayurankar48]

Summary

Closes #233

• Add screen-reader-only CSS class (.bsf-sr-only) and focus outline styles for keyboard navigation
• Add <fieldset>, <legend>, aria-label, and <label> elements to star rating radio buttons in both display_rating() and bsf_display_rating()
• Replace generic image alt text ("1star", "gray", "recipe image", "product image", "event", "software image") with dynamic contextual values from post meta
• Mark decorative star images as aria-hidden="true" with empty alt text
• Add role="complementary" and aria-label to all #snippet-box containers
• Add role="presentation" to the admin meta box layout table
• Add aria-hidden state management to toggle.js show/hide logic
• Refactor toggle.js to use lookup maps, reducing code duplication

Test plan

• Verify star rating widget is navigable via keyboard (Tab/Shift+Tab)
• Test with a screen reader (VoiceOver/NVDA) — star ratings should announce "1 star", "2 stars", etc.
• Confirm snippet box images announce meaningful alt text (product name, recipe name, etc.) instead of generic labels
• Verify focus outlines appear on all interactive elements within snippet boxes
• Check admin meta box schema type selector — hidden sections should have aria-hidden="true", visible sections should not
• Ensure no visual regressions in snippet box rendering across all schema types

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