Improve node rendering and canvas UI quality

[dovvnloading]

Summary

This PR implements the node-rendering and canvas UI QA plan from doc/NODE_RENDERING_UI_QA.md.

What changed

• Fixed prepareGeometryChange() ordering for Document, Thinking, and Connection items.
• Added bounded CodeNode scrolling and generalized wheel routing for scrollable canvas items.
• Added bounded, aspect-preserving ImageNode previews with an invalid-image placeholder.
• Centralized bounded zoom behavior and expanded Fit All to persistent scene item families.
• Made connection painting safe for headless/export scenes with no attached view.
• Reduced interaction overhead with minimal viewport updates, endpoint-indexed connection refreshes, coalesced scene-change notifications, and no stale device-coordinate connection cache.
• Routed canvas typography through shared scene font settings across core nodes, canvas items, and plugin headers.
• Added headless Qt regression coverage for scrolling, image bounds, zoom state, Fit All, no-view rendering, endpoint indexing, and wheel capability routing.
• Added the QA findings/proposals and implementation status document.

Root causes

The main issues were duplicated rendering behavior across QGraphics items, geometry notifications occurring after bounds/path mutation, view-specific assumptions inside scene painting, and interaction paths that scanned or repainted more of the graph than necessary.

Validation

• python -m pytest -q: 401 passed, one existing pytest-asyncio deprecation warning.
• python -m compileall -q .: passed.
• git diff --check: passed.
• Targeted headless canvas suite: 7 passed.

No model/API calls or unrelated files were included.