refactor(automation): drop screenshot.viewport3d, keep only screenshot.window

The on-screen window capture is composited from the desktop framebuffer, so it
already includes the GL 3D viewport as currently shown (model in the editor,
toolpaths in Preview). The offscreen render_thumbnail path only ever drew the
model GLVolumeCollection — never the gcode toolpaths — and produced a blank image
after slicing because the app switches to the Preview panel. Rather than maintain a
second, more limited capture method, remove it entirely.

Removes the JSON-RPC method, IUiBackend/WxUiBackend implementation, dispatcher
route + capability entry, the now-dead opt_int/thumbnail_to_wximage helpers and
ThumbnailData include, the mock override + unit test, and the Python
screenshot_3d client method. Docs updated accordingly.

tests/automation/MockUiBackend.hpp

@@ -18,7 +18,6 @@ public:
     std::vector<std::string> typed_text;
     std::vector<std::vector<KeyChord>> sent_keys;
     int               screenshot_window_count   = 0;
-    int               screenshot_viewport_count = 0;
 
     // Canned outputs (set by tests).
     UiNode   tree;                                  // default tree for dump_tree
@@ -50,10 +49,6 @@ public:
     PngImage screenshot_window(const UiNode*) override {
         ++screenshot_window_count; return canned_png;
     }
-    PngImage screenshot_viewport3d(std::optional<int>, std::optional<int>,
-                                   std::optional<int>) override {
-        ++screenshot_viewport_count; return canned_png;
-    }
 };
 
 }}} // namespace