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.

src/command/cmd_funcs.c

@@ -10633,3 +10633,22 @@ cmd_vcard_save(ProfWin* window, const char* const command, gchar** args)
     cons_show("User vCard uploaded");
     return TRUE;
 }
+
+gboolean
+cmd_force_encryption(ProfWin* window, const char* const command, gchar** args)
+{
+    if (g_strv_length(args) == 1 && (g_strcmp0(args[0], "on") == 0 || g_strcmp0(args[0], "off") == 0)) {
+        _cmd_set_boolean_preference(args[0], "Forces encryption", PREF_FORCE_ENCRYPTION);
+    } else if (g_strv_length(args) == 2 && g_strcmp0(args[0], "policy") == 0) {
+        if (g_strcmp0(args[1], "resend-to-confirm") != 0 && g_strcmp0(args[1], "block") != 0) {
+            cons_show_error("Invalid policy: \"%s\". See '/help force-encryption' for details.", args[1]);
+            return TRUE;
+        }
+        prefs_set_string(PREF_FORCE_ENCRYPTION_MODE, args[1]);
+        cons_show("Force encryption policy has been set to \"%s\".", args[1]);
+    } else {
+        cons_bad_cmd_usage(command);
+    }
+
+    return TRUE;
+}