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 <[email protected]>

src/sys.rs

@@ -1,6 +1,8 @@
 // Small platform shims for things the desktop build does natively but the web
 // (wasm) build must handle differently or skip.
 
+pub mod ui_font;
+
 /// Drawing hyperlinks may open web pages, never local files or custom handlers.
 pub(crate) fn web_hyperlink(value: &str) -> Option<String> {
     if value.chars().any(char::is_control) {