Add an opt-in context menu for files and directories

Apps set browser.contextMenu to a list of { label, action, when?, danger? }
items; when(entry, path) scopes an item to directories, files, or any
other match. Right-click, the Menu key, or a touch long press opens it.
The demo adds sample items, including a .json-only one.

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

demo/assets/index-Bx92WdqS.css

@@ -1 +1 @@
-:root{color:#181c21;font-synthesis:none;font-family:-apple-system,BlinkMacSystemFont,Segoe UI,system-ui,sans-serif}body{min-width:280px;margin:0}main{max-width:1220px;margin:0 auto;padding:20px}@media (width<=640px){main{padding:8px}}
+:root{color:#181c21;font-synthesis:none;font-family:-apple-system,BlinkMacSystemFont,Segoe UI,system-ui,sans-serif}body{min-width:280px;margin:0}main{max-width:1220px;margin:0 auto;padding:20px}@media (width<=640px){main{padding:8px}}.demo-status{color:#57606a;min-height:20px;margin:8px 2px 0;font-size:13px}