feat: TextStyle font browser (STYLE DIALOG command)

- STYLE DIALOG / TEXTSTYLE DIALOG opens the Text Style Font Browser
- Three-panel dialog: style list | built-in CXF font list | properties
- Properties panel: font file text field, width factor, oblique angle
- Apply button commits edits; Set Current sets the active text style
- Built-in font picker (20 CXF fonts from assets/fonts/) auto-applies
  font to the selected style on click
- Preview pane shows "AaBbCc 0123" sample text

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/commands.rs

@@ -2339,6 +2339,9 @@ impl H7CAD {
                 let parts: Vec<&str> = rest.splitn(3, ' ').collect();
                 let sub = parts.get(0).map(|s| s.to_uppercase()).unwrap_or_default();
                 match sub.as_str() {
+                    "DIALOG" | "UI" => {
+                        return Task::done(Message::TextStyleDialogOpen);
+                    }
                     "" | "LIST" | "?" => {
                         let styles: Vec<String> = self.tabs[i].scene.document
                             .text_styles.iter()