Studio: editor-dock layout

- the window is laid out as an editor dock: a tool strip (drawn icons) on the
  left edge, the Explorer, the 3D view, and a tabbed Inspector on the right
  (Selected, World, Environment, Sound, Headset); the timeline and the status
  bar run along the bottom
- the old one-piece properties panel is split into one function per tab;
  selecting something shows the Selected tab
- the Frame menu is now Headset (and opens the Headset panel); the tuning
  tooltip describes the paced walk

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

src/switcher.cpp

@@ -1203,7 +1203,7 @@ struct Panel {
         sc[0].content = envContent();
         if (envs.empty()) {
             const float cy = kTop + (H - kTop) * 0.42f;
-            const std::string a = "No environments yet", b = "Send one from Frame Studio: Frame > Install on Steam Frame.";
+            const std::string a = "No environments yet", b = "Send one from Frame Studio: Headset > Install on Steam Frame.";
             head.draw(cv, (W - head.width(a)) * 0.5f, cy - 40, a, kText);
             small.draw(cv, (W - small.width(b)) * 0.5f, cy + 8, b, kMuted);
             return;