Media: Broaden DIP header to all admin and preview pages

[adamsilverstein]

Summary

• Send Document-Isolation-Policy header on all admin pages (via admin_init) instead of only 4 specific editor screens (load-post.php, load-post-new.php, load-site-editor.php, load-widgets.php)
• Add new wp_set_up_cross_origin_isolation_for_preview() to send DIP on front-end preview pages (via template_redirect)
• This keeps all admin navigations and preview popups in the same browser agent cluster, preserving cross-window communication in Chromium 137+

Why

When the editor sends the DIP header but navigated-to pages (previews, template operations, site editor sub-pages) don't, the browser places them in different agent clusters. This breaks cross-window communication (window.opener, popup references) and currently requires 6 Gutenberg E2E test suites to disable client-side media processing entirely.

Test plan

• Verify PHPUnit tests pass: vendor/bin/phpunit tests/phpunit/tests/media/wpCrossOriginIsolation.php
• Test editor preview in Chrome 137+ — preview popup should communicate with editor
• Test site editor template navigation — no agent cluster mismatch errors
• Test pattern editing — cross-page navigation works
• Verify third-party page builder escape hatch still works (?action=...)
• Verify non-upload-capable users don't get the header

[github-actions]

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