[REFACTOR] Simplify desktop route content primitives after flow redesign

[ksamaschke]

Phase

REFACTOR

## Deliverable
Refine route content primitives, list/detail helpers, and empty-state patterns after GREEN.

## Dependencies
- Parent work item: #217.

## Completion evidence
- Cleanup notes for route content primitives.

• Evidence that the route workflow behavior remains intact.

  Acceptance criteria

  • Route content primitives are maintainable and reusable.

• The cleanup does not regress desktop workflow behavior.

  Parent: Work Item: Redesign Workspace, Sources, Hooks, and Reports as desktop list/detail flows #217

[ksamaschke]

Starting REFACTOR for #217 in worktree /Users/karsten/Work/Development/intelligentcode-ai/intelligent-code-agents-codex-217-route-flows-20260315082619 on branch codex/217-route-flows-20260315082619.\n\nNext step: consolidate the shared desktop list/detail primitives and route selector helpers without widening scope into plugin routes, theming, or acceptance-matrix work.

[ksamaschke]

REFACTOR is complete in the dedicated worktree branch codex/217-route-flows-20260315082619.\n\nCleanup performed:\n- extracted the shared two-pane desktop route shell into src/installer-dashboard/web/src/desktop-master-detail.tsx\n- consolidated repeated route selector buttons into the shared DesktopSelectionRecord helper\n- added shared master/detail and selector styling in src/installer-dashboard/web/src/styles.css while keeping the existing desktop shell and settings boundaries intact\n\nValidation so far:\n- npm run build:quick\n- node --test dist/tests/installer/desktop-route-flow-red-phase.test.js dist/tests/installer/desktop-shell-ux.test.js dist/tests/installer/desktop-source-route-red-phase.test.js dist/tests/installer/desktop-sidebar-routes-red-phase.test.js dist/tests/installer/desktop-settings-window-red-phase.test.js dist/tests/installer/dashboard-source-actions-style.test.js dist/tests/installer/source-management-state.test.js