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/mod.rs

@@ -2460,6 +2460,10 @@ pub enum Message {
     ApplyWebFont(crate::scene::text::web_font::Script),
     /// Completion of the UI renderer's runtime font registration.
     WebUiFontLoaded(crate::scene::text::web_font::Script, Result<(), String>),
+    /// The desktop reported (or a poll may have found) a UI-font change.
+    UiFontSettingsChanged,
+    /// A re-read of the desktop UI font finished.
+    UiFontDetected(Option<crate::sys::ui_font::SystemUiFont>),
     /// Ctrl+V. Routed by `update` into an open text editor, the drawing-object
     /// clipboard, or the system text clipboard.
     PasteShortcut,
@@ -5551,6 +5555,9 @@ use std::path::PathBuf;
 
 #[cfg(not(target_arch = "wasm32"))]
 pub fn run() -> iced::Result {
+    // UI text follows the desktop's interface font: generic sans-serif, iced's
+    // default font, is remapped to it before the first window opens.
+    crate::sys::ui_font::apply(crate::sys::ui_font::detect().as_ref());
     iced::daemon(
         OpenCADStudio::boot,
         OpenCADStudio::update,