Add Italian and the strings every UI key needs

AllStrings.resx was empty, so all 74 UI keys rendered as their own text. It now
holds every key the app uses, with English values. That includes:
- the members of the enums shown through Localizer[value.ToString()], such as
  FollowersOnly and PlainText;
- the new toast, HttpService, ErrorViewer and Settings strings.

AllStrings.it.resx has the same keys in Italian.

ErrorMessages.resx gains MaxLength, with its Designer property, and an Italian
sibling. MessageForm's [StringLength] referenced that key, which didn't exist,
so failing that validation threw at runtime.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw

Resources/ErrorMessages.resx

@@ -117,6 +117,9 @@
   <resheader name="writer">
     <value>System.Resources.ResXResourceWriter, System.Windows.Forms, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089</value>
   </resheader>
+  <data name="MaxLength" xml:space="preserve">
+    <value>The field {0} can be at most {1} characters long.</value>
+  </data>
   <data name="Required" xml:space="preserve">
     <value>This field is required.</value>
   </data>