style(install): trim the new MTU/gate messages to one short line each

Per feedback — shorten the install-failure error (drop the extra hint line) and
the rootless MTU notice.

Signed-off-by: librelad <librelad@digitalangels.vip>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/app/install/app_install.sh

@@ -169,8 +169,7 @@ installApp()
         # still counts; only a total absence is treated as failure.
         if declare -F dockerCommandRun >/dev/null 2>&1 \
            && ! dockerCommandRun "docker ps -a --filter label=com.docker.compose.project=$app_name --format '{{.Names}}' 2>/dev/null" 2>/dev/null | grep -q '[^[:space:]]'; then
-            isError "$app_name did not start — no container was created (usually the image failed to pull). NOT recording it as installed."
-            isNotice "Fix the cause (often image-registry connectivity / MTU), then re-run: libreportal app install $app_name"
+            isError "$app_name: no container started (image pull failed?) — not installed."
             eval "$app_slug=n"
             return 1
         fi

scripts/docker/install/rootless/rootless_docker.sh

@@ -148,7 +148,7 @@ EOF
         # This is the MTU the daemon PULLS over — a blind 1500 on a lower-path-MTU
         # link (Qubes/NAT/VPN) black-holes large image-layer downloads (EOF).
         local resolved_mtu; resolved_mtu="$(networkRedetectMtu)"
-        isNotice "Rootless uplink MTU: $resolved_mtu (CFG_NETWORK_MTU=${CFG_NETWORK_MTU:-auto})"
+        isNotice "Rootless uplink MTU: $resolved_mtu"
 
 sudo bash -c "cat <<EOL > '$override_conf_file'
 [Service]