fix(desktop): surface sidecar startup failures instead of hanging silently (#382)

[lmanchu]

Summary

Fixes the "silent" half of #382: on the packaged desktop app, when the Python sidecar never comes up, the UI renders fully navigable while every backend call hangs forever — no error surfaced anywhere.

Root-cause chain (code-verified on main):

• server_bin() (src-tauri/src/lib.rs) resolves through the sidecar candidates; when spawn() fails, the only signal is an eprintln! — invisible for a Finder-launched app. The shell then continues with no server.
• Even when the spawn succeeds, nothing watches the child: if it exits during startup (bad interpreter, missing dylib, crashed bootloader…) or never binds its port, nothing notices.
• The SPA's boot health polling (App.tsx) burns its ~20s of retries and then presents the normal UI as if everything were fine.

What changed

• Shell (Rust): records sidecar startup state — "starting" → "listening" once the port accepts, or "spawn_failed" / "exited" (with the exit status) — and exposes it via a new get_server_status command. A short-lived watcher thread resolves the state and exits; steady-state cost is zero.
• SPA: when boot health checks fail on desktop, it asks the shell. A dead sidecar fails fast (no 20-second wait) to a full-stop fault screen with the failure detail plus the server log and binary paths; a sidecar that never answers gets the same screen once retries exhaust. The browser build keeps today's behavior.
• The fault screen deliberately outranks the folder gate — nothing behind the gate works without the backend.

This doesn't fix whatever kills a given machine's sidecar (likely several distinct causes behind #382), but it converts each of them from an indefinite silent hang into a visible, diagnosable stop — the log path shown is exactly where the answer lands.

For anyone hitting this today on current builds: ~/.config/coworker/logs/openworker-server.log already captures the sidecar's stdout/stderr and should say why yours died.

Validation

• npx tsc --noEmit — clean
• npx vitest run — 114 passed (3 new, ServerFault component)
• npx playwright test e2e/server-fault.spec.ts e2e/boot.spec.ts e2e/error-retry.spec.ts e2e/smoke.spec.ts — 6 passed (1 new: dead sidecar → fault screen, and the folder gate must not appear)
• cargo check in src-tauri — clean
• No Python changes.

Screenshots

Broken (current main, simulated dead sidecar): the app renders a normal, fully navigable UI; every backend action hangs with no error:

Fixed — sidecar exited during startup:

Fixed — sidecar alive but never answering (retries exhausted):

🤖 Generated with Claude Code