UI polish: sketch snap, sketch/shape list UX, grid settings, docs

[trailcode]

Summary

UI and sketch workflow polish on Trailcode/ui_improve4 (9 commits vs main):

• Sketch snap: simplify try_get_node_idx_snap to axis-guide feedback; restore direct snap to nodes on other visible sketches; fix dimension-tool hover mutating stored node coordinates; remove dead vertex snap annotation code.
• Sketch List: changing the current sketch no longer forces Sketch inspection mode when a sketch tool is already active (e.g. Add line stays active).
• Shape List: hovering a visible shape row highlights that shape in the 3D view; configurable hover color in Settings; pane highlight styling.
• 3D view grid: Settings exposes rectangular grid step and graphic extent / Z offset (occt_view JSON).
• Docs / discoverability: new Sketch snapping section in usage-sketch.md; mid-point snap terminology in usage.md and Add node docs; Sketch List and settings cross-links; GitHub Pages landing page (web/index.html) and sync script; Read the Docs / SEO metadata updates.

Closes #111.

Commits

Commit	Description
66f7f24	Shape List hover drives 3D shape highlight
0bd8936	Shape List pane highlight; grid + hover color settings
eda08e7	SEO / GitHub Pages landing page and doc metadata
4850d84 / d126307	Sketch snap simplification (axis guides)
98dafc6	User guide: sketch snapping section
bae40ae	Agent issue draft (#111)
7c4bd04	Sketch List: preserve active sketch tool when switching sketches
472d88f	Docs: mid-point snap overview in usage.md; aligned Add node wording

Test plan

• Sketch mode: move cursor near nodes — axis guides appear; vertex locks when X and Y align to the same point.
• Dimension tool: hover a node — guides show; node coordinates do not drift.
• Two visible sketches: snap to a node on the other sketch while editing the current one.
• Add node: mid-point snap — click near a straight edge (not at ends); node lands on the segment and the edge splits in two.
• Sketch List: in Add line, switch current sketch — tool stays Add line (not inspection).
• Sketch List: from Normal, switch current sketch — enters Sketch inspection.
• Shape List: hover a visible shape row — shape highlights in 3D; color follows Settings.
• Settings: grid step / extent changes persist and affect the view grid.
• EzyCad_tests.exe --gtest_filter=Sketch_test.* — all pass.