fix: Seahorse does not properly work on many Linux devices

- stop use of `flutter_secure_storage` for Linux in favor of unencrypted
  storage

Unlike many other platforms, many Linux distributions have built-in
support for FDE or home directory encryption. As long as
`flutter_secure_storage` makes FluffyChat useles on many Linuxes, this
seems to be the only solution until they stop using Seahorse.

Signed-off-by: TheOneWithTheBraid <the-one@with-the-braid.cf>

lib/pages/add_story/add_story.dart

@@ -39,11 +39,11 @@ class AddStoryController extends State<AddStoryPage> {
 
   bool get hasMedia => image != null || video != null;
 
-  void updateColors(String text) => hasMedia
+  void updateColors() => hasMedia
       ? null
       : setState(() {
-          backgroundColor = text.color;
-          backgroundColorDark = text.darkColor;
+          backgroundColor = controller.text.color;
+          backgroundColorDark = controller.text.darkColor;
         });
 
   void importMedia() async {

lib/pages/add_story/add_story_view.dart

@@ -98,7 +98,7 @@ class AddStoryView extends StatelessWidget {
                   color: Colors.white,
                   backgroundColor: !controller.hasMedia ? null : Colors.black,
                 ),
-                onChanged: controller.updateColors,
+                onEditingComplete: controller.updateColors,
                 decoration: InputDecoration(
                   border: InputBorder.none,
                   hintText: controller.hasMedia