Desktop: name a default font family, and rebuild fontconfig's cache once

Firefox's own network log shows it fully online - DNS answering for both
families, sockets connecting, HTTP/2 and HTTP/3 sessions carrying data - so
the blank pages were never the network. They are the fonts: Gecko reports
'no fonts' with 13 families found, which is what it says when no generic
family resolves to a face. The linked rules map sans-serif onto faces a full
distribution installs; this rootfs has DejaVu and nothing else, so local.conf
names it outright for every generic, and the cache built into the image is
rebuilt once so the new rules are seen.

tools/linuxfs/desktop/steamdeck-desktop

@@ -44,6 +44,28 @@ if [ -d /usr/share/fontconfig/conf.default ] && ! ls /etc/fonts/conf.d/*.conf >/
   for f in /usr/share/fontconfig/conf.default/*.conf; do ln -sf "$f" /etc/fonts/conf.d/; done
   echo "== fontconfig: linked $(ls /etc/fonts/conf.d/*.conf | wc -l) default rules"
 fi
+# Those rules map sans-serif and friends onto whatever a full distribution installs, and this
+# rootfs has one family. Naming it outright is what gives every toolkit a default font: without
+# one Gecko reports "no fonts" with families found, and draws pages with no text at all.
+if [ ! -f /etc/fonts/local.conf ]; then
+  { echo '<?xml version="1.0"?><!DOCTYPE fontconfig SYSTEM "urn:fontconfig:fonts.dtd"><fontconfig>'
+    for generic in sans-serif sans serif monospace mono cursive fantasy system-ui ui-sans-serif; do
+      case $generic in
+        serif) face="DejaVu Serif" ;;
+        mono*|ui-mono) face="DejaVu Sans Mono" ;;
+        *) face="DejaVu Sans" ;;
+      esac
+      printf '<alias binding="weak"><family>%s</family><prefer><family>%s</family></prefer></alias>\n' "$generic" "$face"
+    done
+    echo '</fontconfig>'
+  } > /etc/fonts/local.conf
+  echo "== fontconfig: wrote local.conf (DejaVu as the default family)"
+fi
+# The cache in the image was built before those rules existed; rebuild it once, and only once.
+if [ ! -f /var/cache/fontconfig/.steamdeck-built ] && command -v fc-cache >/dev/null 2>&1; then
+  fc-cache -f >/dev/null 2>&1 && touch /var/cache/fontconfig/.steamdeck-built
+  echo "== fontconfig: cache rebuilt, default sans is $(fc-match sans-serif 2>/dev/null)"
+fi
 echo "== steamdeck-desktop: labwc ($WLR_RENDERER) on $WAYLAND_DISPLAY"
 # ~/.steamdeck-desktop-debug: labwc's own debug log (views and layer surfaces mapping) too.
 DEBUG=; [ -f "$HOME/.steamdeck-desktop-debug" ] && DEBUG=-d