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/components/Style.cpp

@@ -143,6 +143,10 @@ QString buildAppStyleSheet() {
             font-size: ${font_small}px;
             font-weight: 600;
         }
+        QLabel#panelSectionEmptyLabel {
+            color: ${secondary_text};
+            padding: 4px 10px;
+        }
         QToolButton#panelTitleButton {
             border: none;
             background: transparent;