feat: Add /force-encryption command and update message handling

Implement /force-encryption command to configure forced encryption
settings with on|off and policy resend-to-confirm|block options.

The changes enhance user experience by clarifying encryption requirements
and providing actionable commands. Users can now press Enter again to
confirm unencrypted messages in resend-to-confirm mode.

tests/unittests/ui/stub_ui.c

@@ -675,6 +675,11 @@ inp_readline(void)
     return NULL;
 }
 
+void
+inp_set_line(const char* const new_line)
+{
+}
+
 void
 inp_nonblocking(gboolean reset)
 {
@@ -1157,6 +1162,11 @@ cons_privacy_setting(void)
 {
 }
 
+void
+cons_encryption_setting(void)
+{
+}
+
 void
 cons_show_bookmarks_ignore(gchar** list, gsize len)
 {