Library upload, new folder, rename and delete; a chat's tools and skills (/scope)

Files -> Server writes through /api/v2/files/{upload,mkdir,rename} and
DELETE /files (the trash); upload is a Rust command (library_upload) that
picks files in the system dialog and sends them multipart in the field
`files`. A chat's menu opens Tools and skills..., the web's chat-settings
switches over GET/POST /chats/{id}/scope and /allow/clear. A popover can
open at a context menu's point.

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

tests/ui/mock-tauri.js

@@ -100,8 +100,12 @@
         return ""
       case "terminal_word":
         return "lembas"
-      default:
-        return null
+      default: {
+        // A command the test answers itself: `commands: { name: value }` (an `__error` rejects).
+        const own = (config.commands || {})[cmd]
+        if (own && own.__error) return Promise.reject(own.__error)
+        return own === undefined ? null : JSON.parse(JSON.stringify(own))
+      }
     }
   }