fix(10): resolve CR-01 prop mutation + CR-02 double-shortcut on modal open

CR-01: DocumentCard.vue `@unshared="doc.is_shared = false"` mutated a
defineProps prop (readonly in Vue 3). Replace with a local `isShared` ref
watched against prop changes so the Shared pill hides correctly after revoke.

CR-02: App.vue keyboard handler fired U/N/Escape shortcuts even when a modal
was open (no input focused). Add `role="dialog"` guard so all shortcuts are
suppressed while any dialog is in the DOM.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/App.vue

@@ -30,6 +30,7 @@ function onOsFilesDropped(files) {
 function onKeydown(e) {
   const tag = document.activeElement?.tagName
   if (['INPUT', 'TEXTAREA', 'SELECT'].includes(tag) || document.activeElement?.isContentEditable) return
+  if (document.querySelector('[role="dialog"]')) return
 
   if (e.key === '/' && !e.ctrlKey && !e.metaKey) {
     e.preventDefault()