feat: add multi-language (i18n) support with Japanese UI

crates/ui-egui/src/fonts.rs

@@ -0,0 +1,46 @@
+//! System font loading and fallback setup for multi-language (CJK/Japanese) support.
+
+use std::sync::Arc;
+
+/// Candidate font paths to look for on different operating systems.
+const FONT_CANDIDATES: &[&str] = &[
+    // Windows
+    "C:\\Windows\\Fonts\\meiryo.ttc",
+    "C:\\Windows\\Fonts\\YuGothM.ttc",
+    "C:\\Windows\\Fonts\\YuGothR.ttc",
+    "C:\\Windows\\Fonts\\msgothic.ttc",
+    "C:\\Windows\\Fonts\\BIZ-UDGothicR.ttc",
+    // macOS
+    "/System/Library/Fonts/Hiragino Sans GB.ttc",
+    "/System/Library/Fonts/PingFang.ttc",
+    "/Library/Fonts/Arial Unicode.ttf",
+    // Linux
+    "/usr/share/fonts/opentype/noto/NotoSansCJK-Regular.ttc",
+    "/usr/share/fonts/truetype/noto/NotoSansCJK-Regular.ttc",
+    "/usr/share/fonts/noto-cjk/NotoSansCJK-Regular.ttc",
+    "/usr/share/fonts/truetype/droid/DroidSansFallbackFull.ttf",
+];
+
+/// Configure font definitions with system CJK fallback if available.
+pub fn setup_fonts(ctx: &egui::Context) {
+    let mut font_bytes: Option<Vec<u8>> = None;
+    for path in FONT_CANDIDATES {
+        if let Ok(bytes) = std::fs::read(path) {
+            font_bytes = Some(bytes);
+            break;
+        }
+    }
+
+    let Some(bytes) = font_bytes else {
+        return;
+    };
+
+    let mut fonts = egui::FontDefinitions::default();
+    fonts.font_data.insert("cjk_fallback".to_owned(), Arc::new(egui::FontData::from_owned(bytes)));
+
+    fonts.families.entry(egui::FontFamily::Proportional).or_default().push("cjk_fallback".to_owned());
+
+    fonts.families.entry(egui::FontFamily::Monospace).or_default().push("cjk_fallback".to_owned());
+
+    ctx.set_fonts(fonts);
+}