feat(viewer2d): make the lineweight display scale adjustable from the toolbar

The px-per-mm mapping was a hard-coded guess twice over, and the right
value depends on the monitor - AutoCAD has the same knob as the Adjust
Display Scale slider. Expose it: a slider plus a number box next to the
lineweight toggle, with a readout of what the drawing's own weights come
out to (0.35mm -> 7px, 0.50mm -> 10px at the 20 px/mm default). The
value persists in localStorage.

Retuning applies to the live materials instead of rebuilding the scene.
The hairline cut-off is a millimetre value, so changing the scale moves
every weight by the same factor without changing which bucket a segment
belongs to - each fat batch now remembers its weight in mm, and only
material.linewidth is rewritten. That keeps a drag on the slider instant
even on the 37 MB road drawing.

Measured headless on its 0.35mm gutter line: 2-3px at scale 8, 7px at
20, 14px at 40 - matching the readout in each case.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/viewer2d/Viewer2D.d.ts

@@ -22,6 +22,8 @@ export class Viewer2D {
   /** Lineweight display scale in px per mm (AutoCAD's display-scale slider). Default 20. */
   setLineweightScale(pxPerMm: number | null): void;
   getLineweightScale(): number;
+  /** Distinct lineweights (mm) present in the loaded drawing, ascending. */
+  getLineweightClasses(): number[];
   getLayerInfo(): { name: string; colorHex: string; count: number; visible: boolean }[];
   setHiddenLayers(nameSet: Set<string>): void;
   /** Model / Paper (Layout) spaces for the loaded drawing. */