Fix welcome content padding

[akirk]

Summary

• Add a dedicated empty-state class for welcome/no-posts cards.
• Restore padding and list spacing for welcome content in the default, Mastodon, Twitter, and Google Reader themes.
• Add a dynamic welcome block plus block-theme query-no-results and /friends/?welcome handling.
• Add an unreleased changelog entry.

Testing

• composer check-cs
• php -l templates/frontend/index.php
• php -l templates/mastodon/frontend/index.php
• php -l templates/twitter/frontend/index.php
• php -l templates/google-reader/frontend/index.php
• php -l includes/class-blocks.php
• php -l includes/class-frontend.php
• composer test -- --filter BlockThemeTest (fails: /tmp/wordpress-tests-lib/includes/functions.php is missing)

[github-actions]

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