feat(appearance): fallback font for the custom UI font (#660)

Adds a UI fallback font under Settings › Appearance › Use custom font, so glyphs missing from the chosen UI font (e.g. CJK in Menlo) render in a font the user picks instead of the system default. The fallback joins the UI font stack used by menus, the project tree and dialogs, and resolves macOS user fonts through the shared font loader.

Also keeps the custom font controls wrapping together at narrow widths and covers save/clear/disable, stack composition and settings round-trip in tests.

Fixes #654

Co-authored-by: a-rookie-of-C-language <[email protected]>

src/features/settings/pages/Behavior.module.css

@@ -24,7 +24,7 @@
   padding: 8px 14px;
 }
 
-/* "[x] Use custom font: [Inter] Size: [13]" */
+/* Inline option groups, with custom font controls wrapping together. */
 .fontRow {
   display: flex;
   align-items: center;
@@ -37,8 +37,17 @@
 .fontInput {
   width: 220px;
 }
-.sizeLabel {
-  margin-left: 8px;
+.customFontRow {
+  flex-wrap: wrap;
+}
+.fontOption {
+  display: inline-flex;
+  align-items: center;
+  gap: 8px;
+  max-width: 100%;
+}
+.fontOption > label {
+  flex-shrink: 0;
 }
 .sizeInput {
   width: 64px;