feat(web): curved edges + unique SVG ids for concurrent graph views

Quadratic-bezier edges instead of straight lines, and drop the
auto-refit-on-load that caused a jarring zoom/pan snap once the force
simulation settled. Also namespace each graph's dot-grid pattern id
with a per-instance uuid — multiple SessionGraph instances can now be
mounted at once (one per open task window), and duplicate SVG ids
silently blanked out every graph's background but the first.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/lib/components/GraphBackground.svelte

@@ -198,8 +198,14 @@
       const z = (s.z + tg.z) / 2
       const a = project(s.x, s.y!, z)
       const b = project(tg.x, tg.y!, z)
+      const dx = b.x - a.x
+      const dy = b.y - a.y
+      const len = Math.max(Math.hypot(dx, dy), 1)
+      const curve = Math.min(len * 0.15, 40)
+      const mx = (a.x + b.x) / 2 - (dy / len) * curve
+      const my = (a.y + b.y) / 2 + (dx / len) * curve
       ctx.moveTo(a.x, a.y)
-      ctx.lineTo(b.x, b.y)
+      ctx.quadraticCurveTo(mx, my, b.x, b.y)
     }
     ctx.stroke()