Screenshots: Copy button, right-click menu, and new shots appear on their own

Each screenshot card and the viewer get a Copy button that puts the image on
the clipboard (natively in the desktop app, as PNG in a browser). Right-click
a screenshot to open, copy, save, show it in Finder, or copy its path or name;
right-click the viewer to copy or save. The shelf re-lists the Frame's
screenshots every 8 s while the window is visible and connected, redraws only
when something changed, and keeps thumbnails it already has. Switching
headsets clears the list and ignores answers still on their way.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

ui/frame_host.py

@@ -153,6 +153,19 @@ def open_path(path):
                      stderr=subprocess.DEVNULL, **DETACHED)
 
 
+def reveal_path(path):
+    """Show a file selected in its folder (Linux file managers vary, so there the folder opens)."""
+    path = Path(path)
+    if MAC:
+        cmd = ["open", "-R", str(path)]
+    elif WINDOWS:
+        cmd = ["explorer", f"/select,{path}"]
+    else:
+        return open_path(path.parent)
+    subprocess.Popen(cmd, stdin=subprocess.DEVNULL, stdout=subprocess.DEVNULL,
+                     stderr=subprocess.DEVNULL, **DETACHED)
+
+
 open_url = open_path  # the same openers hand URLs to the default browser