The top-right profile dropdown was a hardcoded HTML block in
admin/header.php, so modules could not add or remove entries and the
plex/watch "settings" links were baked into core.
Move the menu into the NavbarRegistry: CoreNavbarProvider now registers
the core items (edit profile, settings, backups, cache, modules, logout)
under a reserved 'profile' parent, and header.php renders
NavbarRegistry::getChildren('profile') adaptively. There is intentionally
no top-level 'profile' node, so the items never leak into the main
navigation. Order 100–980 is reserved for module-provided links.
Extract NavbarRegistry::collapseDividers() to drop separators orphaned by
permission filtering (leading, trailing, consecutive), and reuse the
shared _xc_nav_visible()/_xc_nav_label() helpers by defining them once
near the top of the header. Remove the hardcoded settings_plex /
settings_watch entries — those now belong to their owning modules.
Add NavbarProfileMenuTest covering collapseDividers, the core 'profile'
contract (no top-level leak, edit first / logout last, permission gates),
and the module-integration contract (reserved order slots, divider
collapse when folder_watch_settings is absent).
