Add Demo Mode for App Store review

[saeedvaziry]

Summary

• Re-implements the iOS native Demo Mode. Toggling Settings → Demo Mode hides real devices and surfaces a seeded "Demo Mac" with two sample projects, mock VCS state, and a simulated terminal.
• All mock traffic is routed through an in-memory DemoBackend that intercepts WSClient.request and emits events through the same bus, so existing call sites (projects, workspace, git store, pane session) are untouched.
• Demo terminal greets with the green "Demo Mode" banner; Enter echoes input and replies with the yellow [Demo Mode] Commands are not executed in demo mode. notice.

Test plan

• Toggle on: Settings → Demo Mode → ON. Real devices disappear; a single "Demo Mac" row appears at 192.168.1.42:4865.
• Tap Demo Mac → projects screen lists muxy and web-app.
• Open muxy → workspace shows two terminal tabs; tapping one shows the green Demo Mode greeting.
• Type a command + Enter in the terminal → echo + yellow notice + fresh prompt.
• Git sheet on muxy: branch main, 1 ahead / 1 behind, one staged + three changed files including untracked docs/demo.md. Stage / unstage / commit / push / pull / switch-branch / create-branch / create-PR all mutate state with simulated delays.
• Worktrees on muxy: main (primary) + feature-search; add/remove updates the list.
• Toggle off: Demo Mac disappears, real devices return; if active device was the demo, it is cleared.
• Persistence: kill + relaunch with demo ON; Demo Mac is still listed.