feat: serve bincio wheel locally for mobile dev testing

- Add GET /api/wheel/download to serve/server.py and edit/server.py:
  serves dist/bincio-*.whl via FileResponse; in production nginx takes
  the request before FastAPI, so this is a no-op there but works locally
- wheel_version response now includes api_url: "/api/wheel/download"
  alongside the nginx-served url field
- Bundle mobile/assets/bincio.whl (built from dist/) as an offline
  fallback for Pyodide testing before the first instance sync
- docs/mobile-app.md: document dev setup — bundled asset, local server
  endpoint, and how to refresh the bundle with uv build + cp

bincio/serve/server.py

@@ -23,7 +23,7 @@ from typing import Any, Optional
 log = logging.getLogger("bincio.serve")
 
 from fastapi import Cookie, FastAPI, File, Form, HTTPException, Request, Response, UploadFile
-from fastapi.responses import RedirectResponse, StreamingResponse
+from fastapi.responses import FileResponse, RedirectResponse, StreamingResponse
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.middleware.gzip import GZipMiddleware
 from fastapi.responses import JSONResponse
@@ -448,9 +448,27 @@ async def wheel_version() -> JSONResponse:
     return JSONResponse({
         "version": version,
         "url": f"/bincio-{version}-py3-none-any.whl",
+        "api_url": f"/api/wheel/download",
     })
 
 
+@app.get("/api/wheel/download")
+async def wheel_download() -> FileResponse:
+    """Serve the bincio wheel directly (used locally; in prod nginx serves /bincio-*.whl)."""
+    import importlib.metadata
+    try:
+        version = importlib.metadata.version("bincio")
+    except importlib.metadata.PackageNotFoundError:
+        version = "0.1.0"
+    wheel_name = f"bincio-{version}-py3-none-any.whl"
+    # Look in dist/ relative to repo root (two levels up from this file)
+    dist_dir = Path(__file__).parent.parent.parent / "dist"
+    wheel_path = dist_dir / wheel_name
+    if not wheel_path.exists():
+        raise HTTPException(status_code=404, detail=f"{wheel_name} not found in dist/")
+    return FileResponse(wheel_path, media_type="application/zip", filename=wheel_name)
+
+
 @app.post("/api/auth/login", response_model=LoginResponse)
 async def login(
     login_req: LoginRequest,