Editor: native folder picker for New/Open Project

Stage-10 editor-UX follow-up ("typing the path by hand is very hard to
use"). Both project dialogs gain a Browse… button that opens the native
folder picker via rfd's xdg-portal backend — pure Rust, one build works
on Wayland and X11 through xdg-desktop-portal. The dialog runs on a
helper thread reporting over an mpsc channel (polled once per frame in
Shell::build), so the editor keeps rendering while the picker is up;
one pick at a time, and the typed path field remains as a fallback for
portal-less environments.

GUI piece — needs an eye-check (on both Wayland and Xorg) before
promotion to main.

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

editor/Cargo.toml

@@ -22,6 +22,9 @@ egui.workspace = true
 egui-wgpu.workspace = true
 egui-winit.workspace = true
 egui_dock.workspace = true
+# Native folder picker for New/Open Project, run on a helper thread so the
+# UI keeps redrawing while the dialog is up (see Shell::poll_folder_pick).
+rfd.workspace = true
 # Editor preferences file I/O reads/writes the same RON shape `Settings`
 # exports; the engine already pulls `ron` in, the editor now does too.
 ron.workspace = true