feat(viewer): Backstage Open/Save modal conform design-spec

Refactor van bestaande Backstage component naar uniforme save-UX
patroon (warmteverlies-template, openaec-orchestrator design-spec
2026-04-16). React/TypeScript + react-i18next + Zustand store.

UI:
- Backstage.tsx: 364→300 regels, knoppen Open/Save/SaveAs/Recent/Close
- Open expand: [Lokaal .wefc] + [Uit Cloud]
- SaveAs expand: [Lokaal export .wefc] + [Naar Cloud]
- Niet-ingelogd → automatisch lokaal export fallback
- AppShell topbar krijgt "Bestand" knop trigger
- 20+ i18n keys toegevoegd in backstage.json (NL)

Out of scope (week 2):
- .wefc envelope parsing/export (backend endpoints ontbreken,
  huidige is placeholder)
- Cloud project state binding (hardcoded "current-project")
- Toast store unificatie

Build: npm build exit 0 (chunk-size warnings only).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jochem25