0021. Desktop is Electron, mobile is Capacitor; the canvas goes native through gpu-core

Status: accepted 2026-10-25. Supersedes ADR 0009 for desktop; ADR 0018 stands for mobile.

Decision

Desktop (macOS, Windows, Linux): Electron shell in apps/desktop over the hosted app. Same routing as the Tauri shell it replaces: FEEGA_URL (default https://oh.feega.app), FeegaNative/<version> user agent, off-origin navigation to the system browser, feega://auth/callback → <app>/auth/callback, single instance.
Mobile (iOS, Android): Capacitor (apps/mobile, ADR 0018).
One web UI everywhere. No Flutter UI, no native screens.
Canvas native through gpu-core (wgpu on Metal / Vulkan / DX12) under the web UI: an Electron native addon on desktop, a Capacitor plugin on mobile. JS owns the document and its ops; it hands gpu-core the display list (ADR 0020) and the viewport; pointer and gestures stay in the page. Frames reach the compositor zero-copy (IOSurface on Apple, shared handle elsewhere).

Identical Chromium UI on every desktop: one engine to test, WebGPU present on all three.
The native canvas removes the WebKit canvas risk that ADR 0009 left unmeasured.

~265–290 MB app, ~3x the memory of a system-WebView shell.