fix(gui): don't crash when the file chooser is in "This PC" on Windows - #3049

Fixes #2548 (also reported in #2586, #2635 and #3029).

On Windows, the Swing file chooser can navigate into shell folders such as "This PC", whose path is ::{20D04FE0-3AEA-1069-A2D8-08002B30309D}. If the dialog is approved while it is in such a folder, for example after picking a drive there, showDialog() calls getCurrentDirectory().toPath(). That throws InvalidPathException: Illegal char <:> at index 0 on the EDT, which is the trace in all four reports. A name typed in that folder fails the same way later in FileUtils.toPathsWithTrim.

Changes in CustomFileChooser:

• showDialog() remembers the current directory only when it has a file system path. Otherwise the previous one is kept.
• approveSelection() checks each selected file (trimmed, as toPathWithTrim does). If one has no file system path, the dialog opens it when it is a folder, or gives the usual error feedback and stays open, instead of returning a path that would crash.
• A small toPathOrNull(File) helper does the conversion.

Real paths behave as before, including names typed with a trailing space. The "Use alternative file dialog" workaround is no longer needed.

Testing:

• CustomFileChooserTest covers the helper. It uses a NUL character so the test fails the same way on every OS.
• On Windows 11 (JDK 24), a small harness took Swing's real "This PC" folder, FileSystemView.getParentDirectory(C:/), a Win32ShellFolder2. Its toPath() throws, and toPathOrNull returns null for it; C:/Users converts normally. I could not click through the dialog itself here.
• ./gradlew clean build dist passes in jadx-gui (78 tests), jadx-cli and spotless. The 7 jadx-core integration tests that fail here (e.g. TestAnonymousClass2) fail because JDK 24's javac warns that -source 8 is obsolete. This change does not touch them.