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/update/mod.rs

@@ -1052,6 +1052,27 @@ impl OpenCADStudio {
                 Task::none()
             }
 
+            #[cfg(not(target_arch = "wasm32"))]
+            Message::UiFontSettingsChanged => Task::perform(
+                async { crate::sys::ui_font::detect() },
+                Message::UiFontDetected,
+            ),
+            #[cfg(target_arch = "wasm32")]
+            Message::UiFontSettingsChanged => Task::none(),
+
+            // Re-setting the defaults recreates the renderers and lays out
+            // every window again with the remapped sans-serif family.
+            #[cfg(not(target_arch = "wasm32"))]
+            Message::UiFontDetected(system) => {
+                if crate::sys::ui_font::apply(system.as_ref()) {
+                    iced::font::set_defaults(iced::Font::DEFAULT, 16.0)
+                } else {
+                    Task::none()
+                }
+            }
+            #[cfg(target_arch = "wasm32")]
+            Message::UiFontDetected(_) => Task::none(),
+
             Message::Tick(t) => self.on_tick(t),
 
             Message::OpenFile => self.on_open_file(),