✨ Add Kotlin/Native Linux file picker via XDG desktop portal

[Enaium]

Summary

Adds file picker, directory picker, and file saver dialogs for Kotlin/Native Linux targets (linuxX64, linuxArm64) to filekit-dialogs, backed by the GNOME XDG Desktop Portal (org.freedesktop.portal.FileChooser).

On GNOME desktops the portal renders the Nautilus-style GNOME file dialog (the native portal is planned to move into Nautilus itself), so this is the Nautilus/GNOME-native picking experience for native Linux apps.

What changed

filekit-dialogs

• Enable linuxX64/linuxArm64 targets in the module convention.
• src/linuxMain: platform actuals for openFilePicker, openDirectoryPicker, openFileSaver, and openFileWithDefaultApplication (via xdg-open), plus FileKitDialogSettings/FileKitOpenFileSettings.
• src/linuxDbusMain: a libdbus cinterop client implementing the portal OpenFile/SaveFile requests (a{sv} options: multiple, directory, filters, current_folder, current_name) and the org.freedesktop.portal.Request.Response signal handling, compiled into both Linux targets (the shared linuxMain metadata compilation cannot see cinterop bindings).
• linuxMain keeps the protocol model and pure helpers (URI decoding, filter building, response resolution) so they are unit-testable.
• Operational failures are wrapped in FileKitPickerException/FileKitDialogException with LINUX_* messages, mirroring the existing Windows/macOS conventions.

Build

• dbus.def cinterop with pkg-config-resolved compiler flags and a -L link path for the distro libdir; consumers link against libdbus-1 (present on every D-Bus enabled system).

Tests

• linuxTest: pure helper tests (URI decoding, filters, response resolution) and failure-mapping tests.
• linuxX64Test: portal response parsing regression tests using real (ua{sv}) D-Bus messages (covers the read path that previously aborted on dbus_message_iter_open_container), a portal connectivity smoke test (skips when no session bus is available), and a manual picker test gated behind FILEKIT_MANUAL_PICKER_TEST=1 that opens the real dialog for click-through verification.

Docs

• docs/installation.mdx and docs/dialogs/setup.mdx updated: dialogs now supported on Kotlin/Native Linux via the XDG portal.

Verification

• ./gradlew assemble and :filekit-dialogs:check pass.
• ktlint clean.
• End-to-end click-through test performed on GNOME (Wayland): dialog opens, multi-select works, paths are returned and printed correctly.
• Note: on this machine running native tests requires a libcrypt.so.1 compatibility shim (pre-existing toolchain/sysroot issue, unrelated to this change).

Affected platforms

• Kotlin/Native Linux (linuxX64, linuxArm64): added.
• filekit-dialogs-compose is unchanged: Compose Multiplatform has no Linux native artifacts yet.

[vinceglb]

Hi @Enaium, thanks a lot for working on native Linux dialog support!

I reviewed the implementation and found three important issues to address before merging:

• assemble currently fails on macOS because the Linux interop requires pkg-config and the D-Bus development headers from the host machine.
• The current_folder option should contain the raw filesystem path. It currently includes a file:// prefix, which can prevent the dialog from opening in the requested directory.
• Percent-encoded paths should be decoded as UTF-8. The current implementation corrupts filenames containing non-ASCII characters such as é.

Can you take a look? Thanks again!

[Enaium]

Thanks for the review! All three issues are addressed:

1. macOS assemble failure — filekit-dialogs now only creates its Linux targets when building on a Linux host (HostManager.hostIsLinux), since the libdbus cinterop requires the host's D-Bus development headers. filekit-core keeps its Linux targets everywhere (it has no cinterop). The Linux CI jobs (build and test-linux) now install libdbus-1-dev so assemble/native tests keep working there.

2. current_folder — now sends the raw filesystem path (null-terminated bytestring) instead of a file:// URI, matching the JVM XdgFilePickerPortal implementation.

3. Percent-decoding — collected bytes are now decoded as UTF-8 (previously each byte was converted to a Latin-1 char, corrupting non-ASCII names like café.png). Added regression tests for percent-encoded and literal UTF-8 paths.

Verified locally: assemble, :filekit-dialogs:check, and the full native test suite (52 tests) pass, and I confirmed via -Dos.name simulation that the Linux targets are absent on non-Linux hosts.

[Enaium]

Publishing has been verified on both platforms without issues:

• macOS: ./gradlew :filekit-dialogs:publishToMavenLocal completes successfully, publishing all variants (jvm, macosArm64, iosArm64, iosSimulatorArm64, mingwX64, js, wasmJs, android), with the configuration cache stored cleanly.
• Linux: the build (assemble) and test-linux (linuxX64Test) CI jobs pass, covering the Linux native targets including the libdbus cinterop.