Web: fetch the CJK font when Japanese text is drawn, whatever the UI language (#720) (#937)

The browser build fetched its Japanese fallback font only for a Japanese UI,
so a Japanese design name, file name or typed text showed as tofu with the UI
in English. After drawing each frame the web shell now looks through the
painted text for kana, CJK ideographs and full-width forms while no CJK font
is loaded, and fetches the same font once on the first hit. A failed fetch
with an English UI leaves the language alone and is not retried in a loop.
Sessions that never show such text download nothing extra.

Fixes #720

Co-authored-by: Claude Opus 5.5 <[email protected]>

apps/solidcraft-web/src/main.rs

@@ -331,22 +331,34 @@ mod web {
                 };
                 if installed {
                     log::info!("loaded web CJK font from {JAPANESE_FONT_PATH}");
-                } else {
+                } else if solidcraft_ui_egui::fonts::needs_cjk_font(&self.app.preferences.language) {
                     log::warn!("CJK font unavailable; showing English for this session");
                 }
                 self.font.finished(installed, &mut self.app.preferences.language);
                 ctx.request_repaint();
             }
             if self.font.tick(&mut self.app.preferences.language) {
-                let slot = self.font_result.clone();
-                let ctx = ctx.clone();
-                wasm_bindgen_futures::spawn_local(async move {
-                    let res = fetch_font_bytes(JAPANESE_FONT_PATH).await;
-                    *slot.borrow_mut() = Some(res);
-                    ctx.request_repaint();
-                });
+                self.fetch_font(ctx);
             }
         }
+
+        /// CJK text on screen with no CJK font yet (an English UI showing a Japanese name):
+        /// fetch the font, so the text does not stay tofu (#720).
+        fn watch_text_for_font(&mut self, ctx: &egui::Context) {
+            if self.font.watching_text() && solidcraft_ui_egui::fonts::painted_cjk(ctx) && self.font.text_needs_font() {
+                self.fetch_font(ctx);
+            }
+        }
+
+        fn fetch_font(&self, ctx: &egui::Context) {
+            let slot = self.font_result.clone();
+            let ctx = ctx.clone();
+            wasm_bindgen_futures::spawn_local(async move {
+                let res = fetch_font_bytes(JAPANESE_FONT_PATH).await;
+                *slot.borrow_mut() = Some(res);
+                ctx.request_repaint();
+            });
+        }
     }
 
     impl eframe::App for Shell {
@@ -365,6 +377,7 @@ mod web {
             let ctx = ui.ctx().clone();
             self.file_panel(&ctx);
             self.recovery_prompt(&ctx);
+            self.watch_text_for_font(&ctx);
         }
         fn save(&mut self, storage: &mut dyn eframe::Storage) {
             // Persist the user's own language, not a session-only English fallback.