fix: smooth freehand stroke capture

[bvnaik05]

Closes #574

What changed

Freehand capture now keeps a more useful distance between stored anchors, with a wider threshold for wider ink. The committed-path rendering contract is unchanged.

Why it matters

Slow strokes retain their intended shape while filtering the dense, low-amplitude samples that make them look mechanical.

Verified

• yarn vitest run src/composables/useWhiteboardInteraction.test.js src/diagram/svgPath.test.js
• yarn eslint src/composables/useWhiteboardInteraction.js src/composables/useWhiteboardInteraction.test.js

Closes #574.

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[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failure remains.

_{Reviews (2): Last reviewed commit: "fix: retain short wide stroke endpoints" | Re-trigger Greptile}

[vibhavkatre]

Sound, and the endpoint guard is the part that makes it safe. Notes from checking it:

• finishStroke has exactly one caller (useWhiteboardInteraction.js:226), so appendStrokeEndpoint can never be handed an undefined endpoint.
• Sparser anchors do not make strokes look angular, because smoothPath draws one quadratic per point through the midpoints — wider spacing rounds the curve rather than faceting it, which is the direction #574 asks for.
• Width-proportional thinning is the right instinct. At the widest highlighter (26) the threshold reaches ~19.5 canvas units, which is coarse in absolute terms; the smoothing hides it for ordinary highlighting, but if a tight highlighter curve ever reads as clipped, that ratio is the dial. Worth remembering it is measured in canvas units, so drawing zoomed in thins more aggressively than drawing at 100%.

Merging.