fix(dev): stop the Tauri dev app from killing concurrently's backend (bun desktop crash) (#745)

`bun desktop` runs concurrently[dev:api, dev:desktop] with --kill-others-on-fail.
dev:api is a uvicorn backend on :3900, but the Tauri app launched by dev:desktop
ALSO manages a backend — on boot it sees :3900 in use (and not yet healthy,
because the dev backend is still importing torch + loading 32 models) and
'takes ownership', killing the dev:api process. That exits 137, which trips
--kill-others-on-fail and tears the whole session down.

The Tauri app already supports TAURI_SKIP_BACKEND to skip backend management
(lib.rs:654) — it just wasn't wired for the concurrently-managed dev flow. Set
it on dev:desktop so the dev app attaches to concurrently's backend instead of
fighting it. Set only on dev:desktop (not dev:api, and not the standalone
`frontend` desktop script, which legitimately self-manages the backend).

bun's script shell evaluates the inline VAR=val cross-platform (verified), so no
cross-env dep / lockfile churn. Prod (desktop-prod) is unaffected — there the
Tauri app is the sole backend manager and orphan-kill is correct.

Co-authored-by: mergetest <test@local>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

package.json

@@ -8,7 +8,7 @@
     "setup:api": "uv sync && uv run python scripts/setup.py",
     "dev:api": "uv run uvicorn main:app --app-dir backend --host 0.0.0.0 --port 3900 --reload --reload-dir backend",
     "dev:frontend": "bun run --cwd frontend dev",
-    "dev:desktop": "bun run --cwd frontend desktop",
+    "dev:desktop": "TAURI_SKIP_BACKEND=1 bun run --cwd frontend desktop",
     "wait:api": "wait-on -t 300000 http-get://localhost:3900/system/info",
     "wait:frontend": "wait-on -t 120000 http://localhost:5173",
     "predev": "kill-port 3900 3901 || true",