Use the xdg-desktop-portal file chooser on Linux

[wienieclaurowy]

Fixes #132.

Wails v2 draws GtkFileChooserDialog directly on Linux, so Aether's file and folder dialogs never reach xdg-desktop-portal and ignore the file picker the user has configured. This routes them through the portal's FileChooser and keeps the Wails dialog as a fallback.

What this changes

• internal/platform/filechooser.go (new): PortalOpenFile calls org.freedesktop.portal.FileChooser.OpenFile over the session bus with godbus (already in the module graph through Wails; go.mod just marks it direct). It subscribes to the Request.Response signal before making the call so a fast reply can't be missed, and also handles the case where the portal returns a handle other than the one derived from handle_token. Wails-style ;-joined patterns are split into portal glob rules. Returned file:// URIs are decoded with url.Parse, so a + in a filename stays a +.
• dialogs.go (new): unexported openFileDialog / openDirectoryDialog helpers on App. On Linux they try the portal first; if the call errors (no portal running, no FileChooser backend), they log it and show the Wails dialog as before. A cancelled pick returns "" just like the Wails dialog. On macOS and Windows nothing changes. The helpers are unexported, so no new frontend bindings are generated.
• app.go: the five existing wailsrt.OpenFileDialog / wailsrt.OpenDirectoryDialog calls go through those helpers. Titles, filters and default directories are passed through unchanged.

No new dependencies.

Behaviour to be aware of

• With Omarchy's default portal config (hyprland;gtk), users get the GTK portal's file chooser instead of the in-process GTK3 dialog. It looks very close, but it is a visible change.
• The portal dialog is not attached to Aether's window as a transient parent, because Wails v2 does not expose the native window handle the portal expects.

Testing

• Added internal/platform/filechooser_test.go: request path derivation, filter splitting, the a(sa(us)) D-Bus signature of the filters, response parsing (selected, cancelled, ended, no URIs, a path with spaces and +), and rejection of malformed bodies and non-file URIs.
• go test ./internal/... ./cli/... passes.
• go vet clean on the changed packages; gofmt -l clean on changed files.
• npm run check: 0 errors. npm test: 116/116 pass (frontend untouched).
• Manual, on Omarchy (Hyprland) with xdg-desktop-portal-termfilechooser as the FileChooser backend: built with make build, launched without GTK_USE_PORTAL / GDK_DEBUG, opened Select Wallpaper and Choose Wallpaper Folder. dbus-monitor showed the portal OpenFile calls answered through the configured backend, and no fallback was logged.

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