feat: font selection with three bundled fonts

- Add FontDef registry (思源黑体 bundled, 霞鹜文楷 + 思源宋体 runtime)
- Runtime font loading from fonts/ dir with fallback to bundled default
- ComboBox in toolbar to switch fonts
- Persist font_name in settings
- Download LXGW WenKai Lite (13MB) and Source Han Serif SC (23MB)

src/app.rs

@@ -57,6 +57,7 @@ impl App {
         let settings_dir = persistence::settings_dir();
         let settings = persistence::load_settings(&settings_dir).unwrap_or_default();
         cc.egui_ctx.set_style(theme::create_style(&settings.theme));
+        crate::font::setup_fonts(&cc.egui_ctx, &settings.font_name);
         Self {
             state: AppState {
                 book: None,
@@ -297,21 +298,23 @@ BgType::Custom(ref path) if !path.is_empty() => {
          let bookmarks = self.settings.bookmarks.get(&file_path).cloned().unwrap_or_default();
          let mut jump_to_bookmark: Option<usize> = None;
 
-         egui::CentralPanel::default().show(ctx, |ui| {
-             let book = self.state.book.as_mut().unwrap();
-             let (action, jump) = crate::reader::reading_view(
-                 ui,
-                 book,
-                 &mut self.state.current_section,
-                 &mut self.state.current_page,
-                 &mut self.state.sidebar_open,
-                 &mut style,
-                 &theme_copy,
-                 bg_type.clone(),
-                 &file_path,
-                 &profile_names,
-                 &bookmarks,
-             );
+          egui::CentralPanel::default().show(ctx, |ui| {
+              let book = self.state.book.as_mut().unwrap();
+              let current_font = self.settings.font_name.clone();
+              let (action, jump) = crate::reader::reading_view(
+                  ui,
+                  book,
+                  &mut self.state.current_section,
+                  &mut self.state.current_page,
+                  &mut self.state.sidebar_open,
+                  &mut style,
+                  &theme_copy,
+                  bg_type.clone(),
+                  &file_path,
+                  &profile_names,
+                  &bookmarks,
+                  &current_font,
+              );
              jump_to_bookmark = jump;
 
              if action.go_back {
@@ -336,7 +339,14 @@ BgType::Custom(ref path) if !path.is_empty() => {
                  }
              }
 
-             if let Some(new_bg) = action.switch_bg {
+              if let Some(ref new_font) = action.switch_font {
+                  if new_font != &self.settings.font_name {
+                      let actual = crate::font::setup_fonts(ui.ctx(), new_font);
+                      self.settings.font_name = actual;
+                  }
+              }
+
+              if let Some(new_bg) = action.switch_bg {
                  self.settings.bg_type = new_bg;
                  // 切换背景时，如果选自定义图片则弹出文件选择器
                  if let BgType::Custom(_) = &self.settings.bg_type {