feat: use aria2c for parallel downloads (8 connections, resumable)

Replaces wget with aria2c -x8 -s8 for significantly faster
downloads, especially on high-latency connections. Falls back to
single-connection if the server doesn't support range requests.

frontend/app.py

@@ -228,10 +228,10 @@ def session_upload(
 
         logger.info("Starting background download: %s → %s", url, dest)
         try:
-            proc = subprocess.Popen(
-                ["wget", "--progress=dot:giga", "-O", str(dest), url],
-                stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL,
-            )
+        proc = subprocess.Popen(
+            ["aria2c", "-x8", "-s8", "-d", str(dest.parent), "-o", dest.name, url],
+            stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL,
+        )
         except Exception as exc:
             return JSONResponse({"phase": "error", "error": f"Failed to start download: {exc}"}, status_code=500)
 

frontend/install.sh

@@ -37,6 +37,7 @@ REQUIRED=(
     python3-pip      "pip3"
     wget             "wget"
     curl             "curl"
+    aria2            "aria2c"
     openssh-client    "scp"
     sshpass           "sshpass"
 )