try: boot's full-page shell for the Social Web screen

[pfefferle]

Spike for the two "Watch" items about our own shell: boot's full-page mode instead of our Layout. Not meant to merge; it is here to look at and decide.

What it does

• loader.ts calls boot's init() instead of initSinglePage(), with menuItems and dashboardLink from PHP (class-app.php, new activitypub_app_menu_items filter), plus an init module (src/app/init.ts) that sets the menu icon (postList from @wordpress/icons) the way boot documents it.
• Our Layout, Sidebar and SiteHub components are deleted (919 lines): boot draws the sidebar, the dashboard back button, the surfaces and the mobile drawer. The feed renders straight into boot's stage column.
• What our sidebar carried moves elsewhere: the feed description, the actor switcher and the settings link into a header at the top of the stage (components/feed-header); the type and tag shortcuts become primary DataViews filters (isPrimary: true), shown next to the search at all times with their options and an "Any" default. The date filter is behind the bar's "Add filter" button.
• The actor switcher inherits the surface's text colour instead of carrying the dark sidebar's.
• The inspector alignment override in style.scss is gone too; boot lays out stage and inspector on its own 8px grid.

Not attempted: generating the PHP with @wordpress/build (CMF-630). That only makes sense if this shell is wanted, and it would turn this into a build-tooling change on top of a UX change.

Findings

1. Mobile has no navigation. Below 782px boot hides its sidebar and expects a page header to render SidebarToggleFill (from @wordpress/admin-ui's Page). That slot lives inside boot's bundled copy of admin-ui, which core does not ship to plugins, so our stage cannot render the toggle and the drawer can never open. No back button either. This is a blocker as long as admin-ui is not exposed.
2. Menu icons need a temporary override on WordPress 7.1. Boot draws its menu entries with components' Item, whose path { fill: currentColor } turns the stroke-based icons of @wordpress/icons 17 into solid boxes. Components 40.1.0 dropped that rule (gutenberg#78808), but WordPress 7.1.x still ships the older wp-components, and boot uses core's copy at runtime. The spike scopes a fill: none override to .boot-navigation-item with a comment to remove it once the minimum WordPress version carries the fix.
3. Desktop works. Sidebar (240px), back button, stage and inspector on boot's grid, feed, search, primary filters and the inspector behave as before.
4. Primary filters disable the funnel toggle by DataViews design (FiltersToggle is disabled when any filter is primary); adding the date filter moves to the "Add filter" button inside the bar.
5. Pre-existing, unrelated: Store "activitypub/app" is already registered in the console. Both the route and the content bundle register the app store (also on trunk). Worth its own fix.
6. Boot's sidebar is navigation only and private: nothing of ours can live in it. With one route it is a 240px column with a single entry.

Testing

• Open the Social Web screen at desktop width, click through the feed, the Type/Tag filters and a post.
• Narrow the window below 782px: the sidebar disappears and nothing brings it back (finding 1).
• The dev site got demo ap_object_type and ap_tag terms assigned to the test posts so the filters have options.