Authoring app architecture design document

[GarthDB]

Context

Part of Epic #732 — Phase 4: Output generators and authoring app integration.

Depends on: #796 (authoring workflow definitions), #797 (wireframes + designer feedback)

Goal

Produce a technical architecture document for the Electron desktop authoring app before any implementation begins. The document should be informed by designer feedback on the wireframes.

Topics to cover

• Runtime: Electron + napi-rs (Node.js bindings to Rust core) — or alternative approaches evaluated
• GitHub authentication: PAT vs OAuth App vs GitHub App — tradeoffs documented
• Multi-repo model: how the app knows which repos to edit, how manifests link them
• Branch/PR workflow: one branch per edit session? batch changes? conflict resolution?
• Real-time validation: how SPEC rules run as the user types (napi-rs FFI into sdk/core)
• Offline/online behavior: local-first editing with deferred push, or always-online?
• Figma relationship: is Figma an input (designers work in Figma, app syncs) or output (app pushes to Figma)?
• Node.js SDK: napi-rs bindings from Phase 5 pulled forward as a dependency

Reference

• tools/component-options-editor/ — existing Figma plugin demonstrating GitHub PAT handling and PR creation patterns
• Figma Variables PoC (Figma Variables output (proof of concept) #795) — informs the Figma integration decision
• Phase 5 Node.js SDK — becomes a dependency of this app

Acceptance criteria

• Architecture document exists at docs/authoring-app/architecture.md
• All 8 topics above addressed with concrete recommendations
• Open questions and decision points flagged for engineering review