Complete Plugin-Driven Admin UI Navigation and Remove Fallbacks

[intel352]

Replace all hardcoded navigation/views in Admin UI with plugin-driven components, killing the FALLBACK_PAGES array and PLUGIN_VIEW_COMPONENTS map.

Tasks

• Refactor core admin views (Dashboard, Editor, Executions, Logs, Marketplace, Templates, Environments, Settings) into NativePlugins.
• Each plugin declares UIPages and RegistersRoutes.
• React UI builds navigation solely from /api/v1/admin/plugins.
• Remove all hardcoded view conditionals from App.tsx.
• Remove FALLBACK_PAGES and pluginStore fallback logic.
• Document required plugin conventions for UI components (registry autoload pattern).

Acceptance

• Admin UI navigation and views entirely driven by enabled plugins.
• No static fallback logic.

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Copilot Agent assigned (can be tackled in parallel).

Linked to broader UI refactor effort.