properties: real empty states + smaller base UI font

- Replace the mock IfcWall placeholder with a "No item selected" empty
  state; the panel only fills in class/attributes/relationships/psets from
  a resolved object, and safely stays empty otherwise.
- Show "No properties" / "No quantities" placeholders (muted, themed via
  secondary_text) when those sets are empty.
- Drop the base application font 10pt -> 9pt to fit more data. Panel titles
  keep their own explicit size and are unaffected.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/bonsaiviewer/main.cpp

@@ -41,7 +41,10 @@ void installUiFont() {
         }
     }
     if (!family.isEmpty()) {
-        QApplication::setFont(QFont(family, 10));
+        // Slightly smaller base font to fit more data. Panel titles keep their
+        // own explicit size (QLabel#panelTitleText in Style.cpp), so they're
+        // unaffected by this.
+        QApplication::setFont(QFont(family, 9));
     }
 }