Keep WordPress theme actions in the first view

[nickhamze]

Summary

• follow up the merged WordPress Themes redesign from Redesign the WordPress Themes workspace #627 with the two issues found in its exact Playground build
• keep the single-theme workspace short enough that Core can focus its details overlay without scrolling the orientation header away
• anchor the active theme actions inside the visible details column
• preserve Core active/inactive action visibility so an active theme no longer exposes Live Preview and Activate controls
• tighten the supporting copy to keep the useful controls in the first window view

Test plan

• npm run build
• npm run lint
• npm run typecheck
• npm run test:js — 362 files / 4,503 tests passed
• vendor/bin/phpcs -n includes/themes-tabs.php tests/phpunit/tests/openStationInjectAppearanceTabs.php
• php -l includes/themes-tabs.php
• php -l tests/phpunit/tests/openStationInjectAppearanceTabs.php
• exact branch Playground QA at the default Appearance window size