Bundle convsim-core into the Tauri desktop app (sidecar)

[charneykaye]

Users should be able to launch a single installer artifact (.dmg / .exe / .AppImage) without running ./scripts/dev.sh. Deferred from alpha (docs/post-alpha-issues.md item 2); the Tauri skeleton already exists in apps/desktop/.

Scope

• Package convsim-core (Python/FastAPI) as a Tauri v2 sidecar started/stopped with the app
• Desktop shell waits for core readiness on 127.0.0.1:7355 before showing UI
• Sidecar lifecycle: clean shutdown, crash restart, port-in-use handling
• All services still bind to 127.0.0.1 only — offline smoke test must pass from the packaged app

Acceptance

• Fresh machine: install → open app → reach the Welcome screen with no terminal involved
• npx convsim offline-smoke-test passes against the packaged build
• CI produces the bundled artifact for at least one platform as a release asset