[treeform]

Summary

• Refresh canvas-relative mouse coordinates before button callbacks, preventing activation clicks from using an old pointer position.
• Capture document-level move/up events for drags that leave the canvas; ignore unrelated unheld document events.
• Reset pointer history on entry and focus loss.
• Add a WASM fixture and browser regression script.

Validation

• Built examples/mouse_drag_regression.nim with Emscripten.
• Ran tests/browser_mouse_drag.js in Chrome: current press position, outside drag movement, outside release, and cleared button state all passed.
• Verified offset-canvas coordinates separately in Chrome.
• Built and launched GotA WASM against this backend.
• Independent review and git diff --check passed.

The similar #198 fixes native macOS button positions; this PR addresses the browser backend.