Fix fontconfig monospace binding overriding app-specific fonts (#5700)

Change monospace match from mode="assign" binding="strong" to
mode="append" binding="same" so the system font acts as a fallback
instead of overriding explicit app font configs like Ghostty's font-family.

Add migration to fix existing user configs.

Fixes #5675

config/fontconfig/fonts.conf

@@ -23,7 +23,7 @@
     <test name="family" qual="any">
       <string>monospace</string>
     </test>
-    <edit name="family" mode="assign" binding="strong">
+    <edit name="family" mode="append" binding="same">
       <string>JetBrainsMono Nerd Font</string>
     </edit>
   </match>

migrations/1778346208.sh

@@ -0,0 +1,11 @@
+echo "Fix fontconfig monospace binding so app-specific fonts are not overridden"
+
+if [[ -f ~/.config/fontconfig/fonts.conf ]] && command -v xmlstarlet >/dev/null 2>&1; then
+  xmlstarlet ed -L \
+    -u '//match[@target="pattern"][test/string="monospace"]/edit[@name="family"]/@mode' \
+    -v "append" \
+    -u '//match[@target="pattern"][test/string="monospace"]/edit[@name="family"]/@binding' \
+    -v "same" \
+    ~/.config/fontconfig/fonts.conf
+  fc-cache -f >/dev/null 2>&1
+fi