feat(statusbar): Select All/Clear All in filter popup, drop osnap settings row

The selection-filter popup now has the same Select All / Clear All header
as the OSNAP popup: Select All clears every exclusion (all types
selectable), Clear All excludes every present type. Each button disables
when it would be a no-op.

Also remove the "Object Snap Settings…" footer from the OSNAP popup.

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

src/app/mod.rs

@@ -1317,6 +1317,10 @@ pub enum Message {
     CloseSelectionFilterPopup,
     /// Include/exclude an entity type from interactive selection.
     ToggleSelectionFilterType(String),
+    /// Make every entity type selectable again (clear the filter).
+    SelectionFilterSelectAll,
+    /// Exclude every present entity type from selection.
+    SelectionFilterClearAll,
     /// Toggle the drawing-units picker open/closed.
     ToggleUnitsPopup,
     /// Close the drawing-units picker.