feat(admin): add plugin management page with settings and dependency warnings (M13) (#14)
Phase 1 plugin management: list installed plugins with enable/disable
toggles, settings modal with auto-generated forms from JSON Schema,
uninstall button, source badges (Core/Official/Community/Experimental),
and dependency warnings when disabling plugins with dependents. Adds
Plugin types, API client functions, mock data, and MSW handlers.
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