Add php-mbstring to installed dependencies

phpSysInfo (Tools -> System Info) requires the mbstring PHP extension.
Without it, the page shows:

  checkForExtensions phpSysInfo requires the mbstring extension to php
  in order to work properly.

Fixes #531

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

scripts/install_services.sh

@@ -17,7 +17,7 @@ install_depends() {
   curl -1sLf 'https://dl.cloudsmith.io/public/caddy/stable/debian.deb.txt' | sudo tee /etc/apt/sources.list.d/caddy-stable.list
   apt -qqq update && apt -qqy upgrade
   echo "icecast2 icecast2/icecast-setup boolean false" | debconf-set-selections
-  apt install --no-install-recommends -qqy caddy sqlite3 php-sqlite3 php-fpm php-curl php-xml php-zip php icecast2 \
+  apt install --no-install-recommends -qqy caddy sqlite3 php-sqlite3 php-fpm php-curl php-xml php-zip php-mbstring php icecast2 \
     pulseaudio avahi-utils sox libsox-fmt-mp3 alsa-utils ffmpeg \
     wget curl unzip bc \
     python3-pip python3-venv lsof net-tools inotify-tools