Skip drive-qualified archive entries instead of failing the restore

Extracting the entry check moved the join to a per-segment form, which
made `C:/evil.txt` slip through: it is not posix-absolute and carries no
`..`, so it became an ordinary segment and produced a path inside the
restore directory that Windows cannot create. Writing it threw an
uncaught FileSystemException and aborted the whole restore, where the
previous inline code simply skipped the entry.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: MOMO0302-02

lib/common/task.dart

@@ -558,8 +558,17 @@ String? resolveArchiveEntryPath(String rootPath, String name) {
       entryPath.startsWith('../')) {
     return null;
   }
+  final segments = posix.split(entryPath);
+  // A drive-qualified segment such as `C:` is not posix-absolute, so it
+  // survives the checks above and would be joined as an ordinary segment.
+  // The resulting path stays inside the root but cannot be created on
+  // Windows, which would abort the whole restore instead of skipping one
+  // entry.
+  if (segments.any((segment) => segment.contains(':'))) {
+    return null;
+  }
   // Join per segment so the result uses the platform separator.
-  final outPath = joinAll([rootPath, ...posix.split(entryPath)]);
+  final outPath = joinAll([rootPath, ...segments]);
   if (!isWithin(rootPath, outPath)) {
     return null;
   }

test/common/task_test.dart

@@ -41,6 +41,18 @@ void main() {
       expect(resolveArchiveEntryPath(root, '/etc/passwd'), isNull);
     });
 
+    // Not posix-absolute, so it reaches the join as an ordinary segment and
+    // yields a path inside the root that Windows cannot create.
+    test('rejects a drive-qualified entry', () {
+      expect(resolveArchiveEntryPath(root, 'C:/evil.txt'), isNull);
+      expect(resolveArchiveEntryPath(root, r'C:\evil.txt'), isNull);
+      expect(resolveArchiveEntryPath(root, 'profiles/C:/evil.txt'), isNull);
+    });
+
+    test('rejects a UNC entry', () {
+      expect(resolveArchiveEntryPath(root, r'\\server\share\evil.txt'), isNull);
+    });
+
     test('rejects backslash traversal', () {
       expect(resolveArchiveEntryPath(root, r'..\evil'), isNull);
       expect(resolveArchiveEntryPath(root, r'profiles\..\..\evil'), isNull);