feat(ui): options panel with an IndexedDB-backed lineweight display scale

Set the default lineweight scale to 11 px/mm: the value the user matched
against AutoCAD on the actual screen. The 20 px/mm derived from their
screenshot was too heavy in practice - the capture and the monitor do
not share a DPI - so the constant is now only a starting point.

Move the lineweight controls off the toolbar into an Options panel:
display toggle, scale slider plus number box, a readout of what the
drawing's own weights come out to, grid toggle, zoom speed, and a reset.
Settings persist in IndexedDB (hmw-viewer/options) rather than
localStorage, so values keep their type and the store has room to grow
per-drawing later; every call degrades quietly if storage is blocked.

Scale changes still apply to the live materials - each fat batch now
remembers its weight in mm - so dragging the slider retunes a 37MB
drawing instantly.

Verified headless: 0.35mm renders 3-4px at scale 11 and 8-9px at 25,
matching the readout, and the value survives a reload via IndexedDB.

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

src/viewer2d/Viewer2D.d.ts

@@ -16,6 +16,8 @@ export class Viewer2D {
   setAccent(hex: string): void;
   setTheme(dark: boolean): void;
   setGrid(visible: boolean): void;
+  setZoomSpeed(speed: number): void;
+  getZoomSpeed(): number;
   /** Lineweight display (AutoCAD LWDISPLAY); null follows the drawing's own flag. */
   setLineweightEnabled(on: boolean | null): void;
   getLineweightEnabled(): boolean;