fix(fonts): repair Waybar font and add system-wide fontconfig defaults

Nerd Fonts renamed "Ubuntu" -> "UbuntuSans"; Waybar's dead family name token-matched to Hack. Correct the explicit name and ship owned conf.d defaults mapping the generic families to the moonarch fonts.

- waybar/style.css: "Ubuntu Nerd Font" -> "UbuntuSans Nerd Font"
- conf.d/65-moonarch-fonts.conf: sans-serif/monospace, binding="strong" (weak prefer ranks behind the generic fallback)
- document fontconfig defaults in CLAUDE.md and DECISIONS.md

defaults/etc/fonts/conf.d/65-moonarch-fonts.conf

@@ -0,0 +1,21 @@
+<?xml version="1.0"?>
+<!-- ABOUTME: System-wide fontconfig generic-family defaults for Moonarch. -->
+<!-- ABOUTME: Loads after 60-latin (number 65) so these prefs win over stock defaults. -->
+<!DOCTYPE fontconfig SYSTEM "urn:fontconfig:fonts.dtd">
+<fontconfig>
+    <description>Moonarch generic-family defaults</description>
+    <!-- binding="strong" required: the default weak <prefer> ranks behind
+         stock generic fallbacks, so it would not take effect. -->
+    <alias binding="strong">
+        <family>sans-serif</family>
+        <prefer>
+            <family>UbuntuSans Nerd Font</family>
+        </prefer>
+    </alias>
+    <alias binding="strong">
+        <family>monospace</family>
+        <prefer>
+            <family>UbuntuSansMono Nerd Font</family>
+        </prefer>
+    </alias>
+</fontconfig>

defaults/xdg/waybar/style.css

@@ -3,7 +3,7 @@
 
 * {
     border: none;
-    font-family: "Ubuntu Nerd Font", sans-serif;
+    font-family: "UbuntuSans Nerd Font", sans-serif;
     font-size: 13px;
     color: alpha(@theme_text_color, 0.8);
 }