fix: #3615293 Match the Share rail's platforms and order to the design, with Instagram as a plain link

Issue: https://www.drupal.org/project/horizonaid/issues/3615293

Declares the Share rail with one setSocialPlatforms config action from Webshare 2.0.0-beta2 (issue #3591909, MR !12 (merged), released): exactly Facebook, Instagram, LinkedIn, X, in the design's order, everything else disabled. Instagram renders as a plain link like the other platforms.

The old simpleConfigUpdate on buttons.* is dropped: Webshare stops consulting that config once its install seeds the webshare_platforms table. No composer change: drupal/webshare already arrives via the required Varbase Blog Base and Varbase Events Base recipes.

Verified live on a Varbase 11 test site with Webshare 2.0.0-beta2 in a real browser: the rail renders Facebook, Instagram, LinkedIn, X in order, all as normal links.

Companion theme MR (Instagram icon in the vartheme_social pack + icon_map): vartheme_bs5_horizonaid!18 (merged)

AI-Generated: Yes

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