fix(flutter): preserve dev-version safety net + update version tests

Two test failures from the comparator rewrite:

1. "different unparseable strings → newer" was useful — branch-name
   builds (e.g. PackageInfo reports 'main' while server reports
   'dev') should still surface the banner so the operator sees that
   something is misaligned. Restore that fallback: if both sides
   parse to all-zero components (no numeric structure on either),
   compare as strings.

2. "honors prerelease ordering" tested pub_semver behavior we no
   longer use. Replaced with tests that cover what the new
   comparator actually does: zero-padding for length mismatch,
   leading-zero normalization, 4-part date+build comparisons, and
   month-rollover correctness without leading zeros.

flutter_client/test/update/client_update_provider_test.dart

@@ -24,17 +24,19 @@ void main() {
       expect(isVersionNewer('v0.1.0', 'v0.1.0'), isFalse);
     });
 
-    test('honors prerelease ordering', () {
-      // 0.1.0 > 0.1.0-rc.1 per semver
-      expect(isVersionNewer('0.1.0', '0.1.0-rc.1'), isTrue);
-      expect(isVersionNewer('0.1.0-rc.1', '0.1.0'), isFalse);
+    test('zero-pads shorter version when comparing', () {
+      // "1.2" treated as "1.2.0" — shorter side gets zero-padded so
+      // comparison is component-wise.
+      expect(isVersionNewer('1.2.1', '1.2'), isTrue);
+      expect(isVersionNewer('1.2', '1.2.1'), isFalse);
+      expect(isVersionNewer('1.2', '1.2.0'), isFalse);
     });
   });
 
-  group('isVersionNewer (date-style versions parse as semver)', () {
-    // pub_semver is permissive with leading zeros — '2026.05.10'
-    // parses as 2026.5.10. So date-style tags get strict ordering,
-    // not the unparseable-fallback path.
+  group('isVersionNewer (date-style versions)', () {
+    // Date tags are 3- or 4-part: 2026.05.10 or 2026.05.10.1. The
+    // numeric comparator parses each component as int (so "05" == 5)
+    // and zero-pads the shorter side.
     test('newer date → newer', () {
       expect(isVersionNewer('2026.05.10', '2026.05.09'), isTrue);
     });
@@ -44,6 +46,18 @@ void main() {
     test('equal date → not newer', () {
       expect(isVersionNewer('2026.05.10', '2026.05.10'), isFalse);
     });
+    test('leading zeros do not affect ordering', () {
+      // "2026.5.11" and "2026.05.11" parse to the same components.
+      // 12 > 5 numerically so month rollover stays correct.
+      expect(isVersionNewer('2026.5.11', '2026.05.11'), isFalse);
+      expect(isVersionNewer('2026.12.1', '2026.5.31'), isTrue);
+    });
+    test('4-part build suffix breaks the tie', () {
+      // 2026.05.10 vs 2026.05.10.1 — first three equal, server has a
+      // 4th component (1), client zero-pads → server is newer.
+      expect(isVersionNewer('2026.05.10.1', '2026.05.10'), isTrue);
+      expect(isVersionNewer('2026.05.10', '2026.05.10.0'), isFalse);
+    });
   });
 
   group('isVersionNewer (truly non-semver fallback)', () {