Build the React consumer over the fail-closed view-mode logic so an admin
can view the app as each of the three homes (User/Admin/SysAdmin) and step
down to a plain User-Side home.
- ViewModeProvider holds the raw requested home (seeded from localStorage,
shape-checked only) and resolves it live against is_admin on every render,
so a demotion or transient /me failure collapses to the User home with no
flash, while an unentitled value is never stored or applied.
- RoleSwitcher renders only for admins (availableViewModes > 1); switching
re-gates the choice and navigates to the chosen home's root.
- AppShell drives its sidebar from sectionsForView(view, isAdmin), which only
ever narrows visibleSections — an admin viewing as a user sees a plain
user's sidebar and lands on the Dashboard at /.
- landingPathForView / viewModeLabelKey added to the logic layer (tested);
view_* and view_switch_label i18n keys added for en-US and zh-CN.
Placement note: the spec calls for a top-right avatar control, but the panel
has no desktop top bar, so the switcher lives in the sidebar foot beside the
user strip. Functionally complete; placement is not yet spec-parity.
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