fix(i18n): add missing note.editMode/viewMode translation keys

Note widget toggle button rendered raw key "widgets.note.editMode"
instead of translated label — keys were never added to either locale.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/web/src/messages/en.json

@@ -200,7 +200,9 @@
       "name": "Notes",
       "description": "Free-text notes with Markdown",
       "defaultTitle": "Note",
-      "autosaveError": "Save failed"
+      "autosaveError": "Save failed",
+      "editMode": "Edit mode",
+      "viewMode": "Preview mode"
     },
     "calculator": {
       "name": "Calculator",