Phase 1 — Host port (abstract the platform)

[ozgesolidkey]

Part of #14. Ref: docs/ARCHITECTURE_V2.md L0.

Wrap window.api behind a Host interface (files, storage, processes, dialogs). Renderer depends on Host, never on Electron directly.

Tasks

• Define Host interface in shared.
• ElectronHost implementation over current IPC.
• Replace direct window.api.* calls in feature modules with host.*.
• (Stub) WebHost over HTTP for later — not wired, just proves the seam.

Done when: no feature module imports window.api directly. Unlocks the web port.