Add native file dialogs for workspace and export paths

A 📂 button beside each path field opens a native picker: a folder
chooser for the workspace and an .odt save dialog for the export (which
appends the extension if omitted). Uses rfd with the XDG Desktop Portal
backend, so there is no GTK build dependency and no new shared library
at runtime.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Cargo.toml

@@ -12,6 +12,8 @@ serde_json = "1"
 pulldown-cmark = { version = "0.12", default-features = false }
 zip = { version = "2", default-features = false, features = ["deflate"] }
 dirs = "5"
+# Native open/save dialogs via the XDG Desktop Portal (no GTK build dependency).
+rfd = { version = "0.15", default-features = false, features = ["xdg-portal", "async-std"] }
 
 [profile.release]
 opt-level = 2