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/reader.rs

@@ -165,6 +165,7 @@ pub struct ReaderAction {
     pub toggle_bookmark: bool,
     pub switch_bg: Option<BgType>,
     pub switch_to_profile: Option<String>,
+    pub switch_font: Option<String>,
     pub page_next: bool,
     pub page_prev: bool,
 }
@@ -181,6 +182,7 @@ pub fn reading_view(
     _file_path: &str,
     profile_names: &[String],
     bookmarks: &[crate::theme::Bookmark],
+    current_font: &str,
 ) -> (ReaderAction, Option<usize>) {
     let mut action = ReaderAction {
         go_back: false,
@@ -188,6 +190,7 @@ pub fn reading_view(
         toggle_bookmark: false,
         switch_bg: None,
         switch_to_profile: None,
+        switch_font: None,
         page_next: false,
         page_prev: false,
     };
@@ -279,6 +282,17 @@ egui::ComboBox::from_id_salt("bg_type_selector")
                                  }
                              }
                          });
+                    egui::ComboBox::from_id_salt("font_selector")
+                        .width(100.0)
+                        .selected_text(current_font)
+                        .show_ui(ui, |ui| {
+                            for name in crate::font::font_display_names() {
+                                let selected = name == current_font;
+                                if ui.selectable_label(selected, &name).clicked() {
+                                    action.switch_font = Some(name);
+                                }
+                            }
+                        });
                     if ui.button("☰").on_hover_text("打开/关闭目录").clicked() {
                         *sidebar_open = !*sidebar_open;
                     }