Add thumbnail toggle and set-all pin-display toolbar icons

Material editor toolbar gets an eye icon to hide/show node thumbnails
(skips the reserved space and decode/render work, not just the
visual) and a 3-dot icon that broadcasts a pin-display mode (All /
Connected / Collapsed) to every node in the open material at once.
The dot icon is shared with the per-node toggle via a parameterized
DrawPinDisplayModeIcon, with dot radius/spacing scaled to fit the
toolbar's fixed icon-button size instead of overflowing it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/ui/MaterialEditorPanel.h

@@ -169,6 +169,8 @@ private:
     bool IsPinVisible(const ShaderGraphNode& node, const std::string& pinName, bool isOutput) const;
     /// Cycles a node's display mode: All -> Connected -> Collapsed -> All.
     void CyclePinDisplayMode(const pxr::SdfPath& nodePath);
+    /// Sets every node currently in m_graph to mode (toolbar "set all" button).
+    void ApplyPinDisplayModeToAllNodes(PinDisplayMode mode);
     /// Authors the node's current PinDisplayOverride into USD customData via
     /// an undoable command, mirroring PersistNodePosition. No-op unless
     /// m_keepGraphNodeViewSettingsInUsd is set.
@@ -249,6 +251,15 @@ private:
     bool            m_openPinPickMenu = false;
     PendingPinPick  m_pendingPinPick;
 
+    /// Toolbar toggle: whether node thumbnails (texture images / material
+    /// shader-ball renders) are shown at all. Off skips the reserved space
+    /// and the decode/render work entirely, not just the visual.
+    bool m_showNodeThumbnails = true;
+    /// Toolbar "set all nodes" pin-display button: the mode last broadcast
+    /// to every node (and the icon/tooltip shown), independent of what any
+    /// individual node is actually in after the user tweaks one by hand.
+    PinDisplayMode m_toolbarPinDisplayMode = PinDisplayMode::All;
+
     /// Column widths of the browser (left) and preview (right) sections,
     /// user-adjustable via the splitters between them and the canvas.
     float m_browserWidth = 220.0f;