Derive line-style icons and edge-detail line from CSS border style

[kmcginnes]

Task

Line styles (solid/dashed/dotted) are currently represented three different ways:

1. Hand-drawn LineSolid/LineDashed/LineDotted SVG icons in the style-dialog dropdowns.
2. A styleForLineStyle() helper in EdgeDetail.tsx that hand-writes gradient CSS for the vertical connector line between the arrows.
3. EdgePreview.tsx, which already renders the real line via borderTopStyle: edgeStyle.lineStyle.

The LineStyle values are valid CSS border-style keywords, so #1 and #2 are hand-rolled duplicates of what CSS gives for free (as demonstrated by #3). Additionally, LINE_STYLE_OPTIONS is defined twice (EdgesStyling/lineStyling.tsx and NodesStyling/lineStyling.tsx), and the NodesStyling copy carries unused render wrappers.

Scope

• Add a LineStyleIcon drawn with borderTopStyle: lineStyle + currentColor (same native mechanism as the edge preview); replace the 3 SVG icons.
• Replace EdgeDetail's styleForLineStyle() gradients with a border-based vertical line; delete the helper.
• Consolidate the two LINE_STYLE_OPTIONS into one { label, value }[] (mirroring ARROW_STYLE_OPTIONS); delete both lineStyling.tsx files.
• Move the icon before the label in the Line Style and Border Style dropdowns (both dialogs), matching the arrow-style dropdowns.
• Delete LineSolid/LineDashed/LineDotted icon files and their barrel exports.

Notes

Follow-up to the arrow-icon dedup (#1985); same single-source-of-truth pattern.

Important

Internal only — this issue is maintained by the core team and is not accepting external contributions.