Cleanup Legacy UI and Remove Deprecated Scaffolding

[intel352]

Sweep and remove all remaining legacy UI code, fallback mechanisms, embedded asset logic, and temp/stale navigation logic after the completion of the new admin UI decoupling and plugin-driven navigation issues.

Tasks

• Remove any code marked FALLBACK, TEMPORARY, or legacy from workflow/ui and backend.
• Purge embed logic and related helpers for UI in Go modules.
• Remove static directory assets and any remaining special-case wiring for static.fileserver, admin UI, or legacy React code.
• Update docs, onboarding material, and migration guides for the new approach.
• Ensure no navigation, menu, or plugin scaffolding remains outside the dynamic plugin-driven system.
• Scan for duplicated or vestigial code between admin UI and extracted component library.

Minimum Checklist (code/files/sections known to be affected)

• FALLBACK_PAGES array — ui/src/store/pluginStore.ts
• PLUGIN_VIEW_COMPONENTS map — ui/src/App.tsx
• Hardcoded view conditionals in App.tsx (Dashboard, Editor, Executions, etc.)
• Hardcoded navigation categories in ui/src/components/navigation/AppNav.tsx
• //go:embed all:ui_dist directive — module/api_workflow_ui.go
• module/ui_dist/ directory (embedded admin UI assets)
• admin.WriteUIAssets() — admin/admin.go
• admin.InjectUIRoot() — admin/admin.go
• mergeAdminConfig() UI extraction logic — cmd/server/main.go
• module.ExtractUIAssets() — module/api_workflow_ui.go
• deriveEnabledPages() fallback merge logic — ui/src/store/pluginStore.ts
• All comments marked TEMPORARY or Remove once all plugins implement UIPages() — ui/src/store/pluginStore.ts
• Documentation referencing legacy embed/build/fallback navigation — docs/DEPLOYMENT_GUIDE.md, docs/tutorials/building-apps-with-workflow.md, docs/ADMIN_PLUGIN_ARCHITECTURE.md

Acceptance

• No references to fallback, temporary, or deprecated admin UI logic remain.
• All navigation and UI serving uses the new plugin-driven system.
• Documentation is current and references only the new process.

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This issue is BLOCKED by completion/merge of #34 and #35.