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

@@ -229,7 +229,7 @@ def session_upload(
         logger.info("Starting background download: %s → %s", url, dest)
         try:
         proc = subprocess.Popen(
-                ["wget", "--progress=dot:giga", "-O", str(dest), url],
+            ["aria2c", "-x8", "-s8", "-d", str(dest.parent), "-o", dest.name, url],
             stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL,
         )
         except Exception as exc:

frontend/install.sh

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