Network Admin: Migrate to unified AdminPage header (#47912)

* Network Admin: Add React entry point for unified header

Add network-admin.tsx entry point that renders the AdminPage component
from @automattic/jetpack-components around the server-rendered network
admin page content. Includes Sites/Settings navigation buttons in the
header actions slot and proper content spacing via Container.

Add the network-admin entry to the Jetpack plugin webpack config.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Network Admin: Replace legacy masthead with unified AdminPage header

Replace Jetpack_Admin_Page::wrap_ui() on both Network Sites and Network
Settings pages with the new React-based AdminPage wrapper. The PHP page
content is rendered in a hidden div and relocated into the React tree
at mount time, preserving all existing functionality.

- Remove old wrapper style hooks (dops-css, genericons)
- Add admin_init_network_page() to enqueue the new JS/CSS bundle
- Localize Sites/Settings navigation URLs for the header action buttons
- Remove network_admin_page_header() calls (old PHP masthead)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Use sentence case in page names

Co-authored-by: Mikael Korpela <mikael@ihminen.org>

* Clean out contents from the page

* Replace site-specific links with generic support link in the footer

* Remove buttons

* changelog

* fix types

* Update var names and remove manual dependency

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Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Co-authored-by: Mikael Korpela <mikael@ihminen.org>
Co-authored-by: Mikael Korpela <mikael.korpela@automattic.com>

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/23947042073

Upstream-Ref: Automattic/jetpack@41f2251

Author: CGastrell