fix(gui): Remove spaces from the Pro token input field (#875)

Sometimes copy-pasting a pro token comes with trailing spaces. Neither
leading, trailing or even middle spaces are alloweable in a token. This
PR leverages Flutter builtin mechanisms to filter input supplied to the
token text field such that any kind of whitespaces (not only the ASCII
ones) are discarded before any further processing.

Closes #857 

---

I'm purposefully basing this PR on top of #873 so I can stress CI to
exercise the changes therein introduced. There is no dependency between
the two tasks.

---
UDENG-3810

gui/packages/ubuntupro/lib/pages/subscribe_now/subscribe_now_widgets.dart

@@ -1,5 +1,6 @@
 import 'package:dart_either/dart_either.dart';
 import 'package:flutter/material.dart';
+import 'package:flutter/services.dart';
 import 'package:flutter_gen/gen_l10n/app_localizations.dart';
 import 'package:yaru/yaru.dart';
 import '../../core/either_value_notifier.dart';
@@ -78,6 +79,10 @@ class _ProTokenInputFieldState extends State<ProTokenInputField> {
           children: [
             Expanded(
               child: TextField(
+                inputFormatters: [
+                  // This ignores all sorts of (Unicode) whitespaces (not only at the ends).
+                  FilteringTextInputFormatter.deny(RegExp(r'\s')),
+                ],
                 autofocus: false,
                 controller: _controller,
                 decoration: InputDecoration(

gui/packages/ubuntupro/test/pages/subcribe_now/subscribe_now_widgets_test.dart

@@ -137,6 +137,24 @@ void main() {
         final input = tester.firstWidget<TextField>(inputField);
         expect(input.decoration!.errorText, isNull);
 
+        final button =
+            tester.firstWidget<ElevatedButton>(find.byType(ElevatedButton));
+        expect(button.enabled, isTrue);
+      });
+      testWidgets('good token with spaces', (tester) async {
+        await tester.pumpWidget(theApp);
+        final inputField = find.byType(TextField);
+
+        await tester.enterText(
+          inputField,
+          // good token plus a bunch of types of white spaces.
+          ' ${tks.good} \u{00A0}\u{2000}\u{2002}\u{202F}\u{205F}\u{3000} ',
+        );
+        await tester.pump();
+
+        final input = tester.firstWidget<TextField>(inputField);
+        expect(input.decoration!.errorText, isNull);
+
         final button =
             tester.firstWidget<ElevatedButton>(find.byType(ElevatedButton));
         expect(button.enabled, isTrue);