Follow the system UI font face

Detect the desktop's configured interface font and use its family for all
application UI text, instead of iced's generic sans-serif.

- sys::ui_font reads the setting per platform: GNOME/Cinnamon/MATE
  gsettings, KDE kdeglobals, Xfce xfconf, the XDG Settings portal, GTK
  settings.ini and a Fontconfig fallback on Linux; the non-client message
  font (SystemParametersInfoForDpi at 96 DPI) on Windows; the AppKit system
  font on macOS.
- ui_font::apply remaps generic sans-serif in iced's shared font database
  to that family. iced's Font::DEFAULT is sans-serif, so every widget,
  canvas text, bold/italic variant and ribbon measurement follows it with
  no call-site changes. A family that is not installed (or a generic alias)
  keeps iced's own choice. The font's weight and style are ignored.
- Live updates: portal SettingChanged on Linux, a 2 s poll on Windows. A
  change bumps the font-system version so laid-out text reshapes, and
  iced::font::set_defaults relays out every window; ribbon large-button
  widths re-measure. Without a portal, Linux needs a restart; macOS has no
  user-adjustable UI font.
- Monospaced text, drawing text (own cosmic-text font systems) and the web
  build's font loading are unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/app/view/mod.rs

@@ -146,6 +146,22 @@ pub(in crate::app) const VIEWPORT_CAPTURE_BOUNDS_ID: &str = "viewport-capture-bo
 
 const VIEWCUBE_HIT_SIZE: f32 = VIEWCUBE_REGION_PX;
 
+/// Desktop UI-font changes while running. Linux listens for the Settings
+/// portal's `SettingChanged` (no portal: restart to apply). Windows exposes no
+/// change notification through iced, but the metrics read is a cheap system
+/// call, so it is polled. macOS has no user-adjustable UI font, and the web
+/// build follows the font it loads for the UI language.
+fn ui_font_changes() -> Subscription<Message> {
+    #[cfg(target_os = "linux")]
+    return Subscription::run(crate::sys::ui_font::change_signals)
+        .map(|()| Message::UiFontSettingsChanged);
+    #[cfg(target_os = "windows")]
+    return iced::time::every(std::time::Duration::from_secs(2))
+        .map(|_| Message::UiFontSettingsChanged);
+    #[allow(unreachable_code)]
+    Subscription::none()
+}
+
 /// Background used by drafting overlays in model or paper space.
 fn crosshair_background(tab: &DocumentTab, is_paper: bool) -> [f32; 4] {
     if !is_paper {
@@ -3052,6 +3068,7 @@ impl OpenCADStudio {
             single_instance,
             hatch_pattern_keys,
             keyboard_events,
+            ui_font_changes(),
         ])
     }