fix(mullvad): Use --data-urlencode for Mullvad API key registration

Base64 WG pubkeys with + or / chars break plain -d POST. Hit this
on VPS setup — "Invalid public key" from Mullvad API.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

files/usr/bin/parahub-mullvad

@@ -95,8 +95,8 @@ cmd_setup() {
     # --- Step 2: Register with Mullvad API ---
     echo "Registering key with Mullvad..."
     RESULT=$(curl -s --max-time 15 -X POST https://api.mullvad.net/wg/ \
-        -d "account=$ACCOUNT" \
-        -d "pubkey=$PUBKEY")
+        --data-urlencode "account=$ACCOUNT" \
+        --data-urlencode "pubkey=$PUBKEY")
 
     if echo "$RESULT" | grep -q "^[0-9]"; then
         MULLVAD_IPV4=$(echo "$RESULT" | cut -d',' -f1)