Fix #81: Chinese, Japanese and Korean names show in the UI, not boxes

Layer and object names, document tabs and any other UI text in scripts the
UI fonts lack now draw in an installed font that covers them, as the canvas
already did.

Root cause: egui only had the bundled UI fonts (Source Sans 3, JetBrains
Mono) and its defaults, none with CJK glyphs. After each frame the UI now
looks at the text it painted; the first time a character the UI fonts lack
appears, a thread finds an installed font covering it through the font
database (the canvas's fallback search, so a font the canvas loaded is
reused, its bytes shared rather than copied) and adds it to every egui
family as the last fallback. Nothing is bundled, Latin-only UIs load no
extra font, and on the web, without system fonts, the boxes remain.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/text/src/fontdb.rs

@@ -675,6 +675,13 @@ impl FontDb {
         None
     }
 
+    /// A face that covers `c`, for text drawn outside the canvas (the app's own UI): the loaded
+    /// fonts first, then (native) an installed one, loaded on demand. Characters no font covers
+    /// are remembered, so they are looked for once.
+    pub fn face_covering(&self, c: char) -> Option<Arc<FontFace>> {
+        self.fallback_for(c, 0)
+    }
+
     fn loaded_fallback(&self, c: char, exclude: u32) -> Option<Arc<FontFace>> {
         let faces = self.read_faces();
         let mut order: Vec<&Arc<FontFace>> = faces.iter().filter(|f| f.id != exclude).collect();