Add a Properties view for the app root

The root tree row now names the app component, "app (<host:port>)" or
"app (<file>)", and offers Properties from its context menu and on
double-click. The live DOM's <app> element now carries its type and slot
values, and the refresh-by-id lookup finds it, so the view shows real
sys::App values.

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

properties.py

@@ -508,7 +508,9 @@ class PropertiesPanel:
         # the view would open already claiming unsaved edits.
         self._loading = False
 
-        comp_type = comp.get("type", "sys::Component")
+        # A .sax file's <app> carries no type attribute: it is always a sys::App.
+        default_type = "sys::App" if comp.tag == "app" else "sys::Component"
+        comp_type = comp.get("type") or default_type
         self.slots = schema.get_full_slots_for_type(comp_type)
 
         self.frame = tk.Frame(parent)
@@ -624,7 +626,8 @@ class PropertiesPanel:
         name_font = tkfont.Font(family="Segoe UI", size=9, weight="bold")
         type_font = tkfont.Font(family="Segoe UI", size=9)
 
-        self._tinted(tk.Label(header, text=self.comp.get("name", "?"), font=name_font,
+        default_name = "App" if self.comp.tag == "app" else "?"
+        self._tinted(tk.Label(header, text=self.comp.get("name", default_name), font=name_font,
                               bg=HEADER_BG), HEADER_BG).pack(side=tk.LEFT)
         self._tinted(tk.Label(header, text=f"({comp_type})", font=type_font,
                               fg="#555555", bg=HEADER_BG),