feat: #3614514 Add the Instagram share icon to the social pack

Issue: https://www.drupal.org/project/vartheme_bs5_horizonaid/issues/3614514

The Horizon Aid recipe adds an Instagram entry to the Share rail as a plain link (Horizon Aid issue #3615293, MR horizonaid!39 (merged)), but the theme's vartheme_social icon pack had no Instagram icon and the theme's Webshare icon_map had no instagram entry, so the button fell back to the generic glyph.

Adds images/icons/social/instagram.svg (Bootstrap Icons glyph, brand fill, same pattern as the pack's other icons) and the instagram icon_map entry in config/optional/webshare.settings.yml. No script changes: the share buttons stay plain links.

Verified live in a real browser on a Varbase 11 test site: the rail renders Facebook, Instagram, LinkedIn, X in the design's order.

AI-Generated: Yes

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