feat(workspace): focus mode for notes

Ctrl+Shift+F or the note menu shows only the note over the whole window
at reading width; the header (title, actions, leave) slides in at the
top edge, Escape leaves; phones get a round exit button.

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

frontend/src/index.css

@@ -571,3 +571,15 @@ fieldset:disabled :is(button, input, select, textarea) {
     --outline-min-text: 0px;
   }
 }
+
+/* Focus mode: the note fades in over the app */
+@keyframes focus-mode-in {
+  from { opacity: 0; transform: scale(0.99); }
+  to { opacity: 1; transform: none; }
+}
+.focus-mode-enter {
+  animation: focus-mode-in 300ms ease-in-out;
+}
+@media (prefers-reduced-motion: reduce) {
+  .focus-mode-enter { animation: none; }
+}