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>

PLAN.md

@@ -680,6 +680,9 @@ console)**. Each is GUI → `dev` + eye-check.
   path, which the maintainer confirms "is very hard to use". Wanted: a native
   file/folder picker (a `rfd`-style dialog, working on Wayland + X11) and/or a
   better-designed in-editor chooser, instead of a free-typed path field.
+  **🚧 On `dev` (2026-07-10), awaiting eye-check**: Browse… buttons on both
+  dialogs → native folder picker (`rfd`/xdg-portal, Wayland + X11) on a helper
+  thread; the typed field remains as fallback.
 - **"New Script" button on the `Script` component inspector** (Stage 10
   follow-up). Create a new `.rhai` from a template into `assets/scripts/` and
   auto-assign it to the component, without leaving the editor — today scripts