fix: even out the pen/highlighter middle size dot

[bvnaik05]

Problem

The pen and highlighter size pickers show three preview dots (small/medium/large) meant to be visibly, proportionally distinct. In practice, the middle dot barely differed from the small one — small and medium read as the same size, with the only real jump landing between medium and large.

Root cause

dotStyle scaled a size's preview dot by that size's value position within the row's own range: (size - smallest) / (largest - smallest).

The pen row's widths are [2, 4, 8] — not evenly spaced (gaps of 2 and 4). The middle value (4) sits only a third of the way between 2 and 8, so its dot landed close to the small dot's diameter, with the real jump saved for the small→large or medium→large step. The highlighter row ([10, 18, 26]) happens to be evenly spaced, so it didn't show the same defect as visibly — which is part of why this survived a first pass at #498.

Fix

Scale by index in the row instead of by value. Every row always has exactly three options, so three evenly-stepped diameters (min, midpoint, max) read as small/medium/large regardless of how far apart the underlying widths happen to be — order is preserved (the arrays are already ascending), just the step size is now even.

Extracted dotStyle out of WhiteboardTools.vue into frontend/src/diagram/sizeDot.js so it has a real, importable unit test covering the exact case that broke — sizeDots.test.js used to carry a hand-restated copy of the scaling logic because the .vue file wasn't importable from the browser-free test env; it now imports the real function instead of maintaining a parallel copy that could drift.

Testing

• yarn vitest run — 150 files / 1735 tests pass.
• New sizeDot.test.js: asserts the middle option is evenly between min/max regardless of value spacing, and pins the exact pen-vs-highlighter case that motivated the fix.
• sizeDots.test.js updated to import the real dotDiameter rather than restate it.

🤖 Generated with Claude Code

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[vibhavkatre]

Verified against the source. Merging.