[treeform]

Assigning CustomCursor on Emscripten previously reset the browser cursor to auto. Encode the supplied image as a PNG data URL and apply its hotspot through the canvas CSS cursor property. A nil image keeps the default cursor, and image encoding failures are reported as WindyError.

Use Pixie's existing encodeImage(PngFormat) API with standard base64 encoding so the change remains compatible with the pinned Pixie 5.1.0 dependency. The string passed to JavaScript now uses an explicit cstring conversion.

Validation:

• nim check --threads:off -d:emscripten examples/custom_cursor.nim passed.
• nim c --threads:off -d:emscripten examples/custom_cursor.nim passed with Nim 2.2.10 and Emscripten 6.0.1.
• Ran the compiled example in the browser: the initial 32-by-32 PNG cursor uses hotspot (0, 0), one left click switches to auto, and the second restores the custom cursor. No browser warnings or errors.
• git diff --check passed.