v0.27.1:0 - fix model download: prepend ~/.local/bin so SSH finds uvx

hf-download.sh shells out to uvx (the uv installer drops it in ~/.local/bin),
but the non-interactive SSH session doesn't source the user's profile, so
~/.local/bin was off PATH and downloads died with "uvx: command not found".
build_download_command now prepends $HOME/.local/bin. Adds test_download.py.

image/app/download.py

@@ -23,6 +23,20 @@ from .ssh import ssh_stream, StreamHandle
 Mode = Literal["spark1", "spark2", "cluster"]
 
 
+def build_download_command(repo: str, flags: str = "") -> str:
+    """Remote shell command that drives hf-download.sh on a Spark.
+
+    Prepends ~/.local/bin to PATH. hf-download.sh shells out to `uvx` (Astral's
+    uv), and the official uv installer drops its binaries in ~/.local/bin — but
+    our SSH session is non-interactive, so it never sources the user's profile
+    and ~/.local/bin is off PATH, leaving `uvx` as "command not found". $HOME
+    expands server-side, so this stays correct for any adopter/user. `repo` is
+    shlex-quoted at the sink (validate_repo gates the charset upstream).
+    """
+    serve = f"./hf-download.sh {quote_arg(repo)} {flags}".strip()
+    return f'export PATH="$HOME/.local/bin:$PATH" && cd ~/spark-vllm-docker && {serve}'
+
+
 _TQDM_RE = re.compile(
     r"(\d+(?:\.\d+)?)\s*%\s*\|.*?\|\s*"
     r"([\d.]+[KMG]?B?)\s*/\s*([\d.]+[KMG]?B?)\s*"
@@ -126,7 +140,7 @@ class DownloadManager:
         if not target_host or not target_user:
             raise RuntimeError(f"{job.mode} host not configured")
 
-        cmd = f"cd ~/spark-vllm-docker && ./hf-download.sh {quote_arg(job.repo)} {flags}".strip()
+        cmd = build_download_command(job.repo, flags)
         job.append(f"$ {cmd}")
         job.state = "downloading"
         job.progress.phase = "Connecting to Hugging Face…"