gtk: restore flatpak-aware resource directory support (#8816)

This was not ported to gtk-ng before old runtime was removed, breaking
shell integration on Flatpak.

This implementation is copied verbatim from old runtime.

src/apprt/gtk.zig

@@ -3,7 +3,7 @@ const internal_os = @import("../os/main.zig");
 // The required comptime API for any apprt.
 pub const App = @import("gtk/App.zig");
 pub const Surface = @import("gtk/Surface.zig");
-pub const resourcesDir = internal_os.resourcesDir;
+pub const resourcesDir = @import("gtk/flatpak.zig").resourcesDir;
 
 // The exported API, custom for the apprt.
 pub const class = @import("gtk/class.zig");

src/apprt/gtk/flatpak.zig

@@ -0,0 +1,29 @@
+const std = @import("std");
+const Allocator = std.mem.Allocator;
+const build_config = @import("../../build_config.zig");
+const internal_os = @import("../../os/main.zig");
+const glib = @import("glib");
+
+pub fn resourcesDir(alloc: Allocator) !internal_os.ResourcesDir {
+    if (comptime build_config.flatpak) {
+        // Only consult Flatpak runtime data for host case.
+        if (internal_os.isFlatpak()) {
+            var result: internal_os.ResourcesDir = .{
+                .app_path = try alloc.dupe(u8, "/app/share/ghostty"),
+            };
+            errdefer alloc.free(result.app_path.?);
+
+            const keyfile = glib.KeyFile.new();
+            defer keyfile.unref();
+
+            if (keyfile.loadFromFile("/.flatpak-info", .{}, null) == 0) return result;
+            const app_dir = std.mem.span(keyfile.getString("Instance", "app-path", null)) orelse return result;
+            defer glib.free(app_dir.ptr);
+
+            result.host_path = try std.fs.path.join(alloc, &[_][]const u8{ app_dir, "share", "ghostty" });
+            return result;
+        }
+    }
+
+    return try internal_os.resourcesDir(alloc);
+}