fix T-287: silence GTK file-chooser GLib-GIO-CRITICAL noise

Root cause (confirmed by standalone reproducer): the Open Workspace
folder picker. GTK's GtkPlacesSidebar enumeration inside
gtk_file_chooser_dialog_new(SELECT_FOLDER) builds GFileInfo objects
without standard::size, then calls g_file_info_get_size() — a
GTK-internal bug that emits a GLib-GIO-CRITICAL pair on every pick.
Ruled out the T-138 clipboard channel.

Two-part fix:
- Switch pickDirectory to GtkFileChooserNative (portal-backed; runs
  out-of-process and silently in sandboxed/Flatpak builds).
- Install a narrowly-scoped GLib-GIO log handler that drops only the
  "g_file_info_get_size / without standard::size" message and forwards
  every other GLib-GIO critical untouched — the universal fix for the
  non-sandbox fallback path.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -247,6 +247,12 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
 
 ### Fixed
 
+- **Open Workspace no longer spews `GLib-GIO-CRITICAL` to the console.** The
+  folder picker now uses the portal-backed `GtkFileChooserNative` (out-of-process
+  in sandboxed/Flatpak builds), and a narrowly-scoped GLib log filter swallows the
+  known-benign `g_file_info_get_size … without standard::size` message GTK's
+  file-chooser sidebar emits internally on every pick — every other GLib-GIO
+  critical still surfaces. (T-287)
 - **The conversation re-anchors when the input area resizes.** Opening a
   permission prompt or AskUserQuestion (which grows the bottom zone, D-78) no
   longer hides the last message behind it — when pinned to the tail, the view