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>

Cargo.toml

@@ -45,6 +45,10 @@ gltf = { version = "1.4", features = ["utils"] }
 ab_glyph = "0.2"
 
 # Editor UI (egui — integrated into oxide-editor only)
+# Native file/folder dialogs (New/Open Project). The default `xdg-portal`
+# backend is pure Rust and talks to xdg-desktop-portal over D-Bus, so one
+# build serves both Wayland and X11 with no GTK link-time dependency.
+rfd = "0.15"
 egui = "0.34"
 egui-wgpu = "0.34"
 egui-winit = "0.34"