feat(handtracking): redesign SVG hand as primary visualization

Rewrite the live hand visualizer as a light-blue silhouette with a
crisp dark-blue outline, suitable as the primary hand UI (replacing
the buggy 3D glove for the default flow):

- Palm polygon (landmarks 0,1,5,9,13,17) and five finger tubes merged
  via an SVG feMorphology filter, so the outline is a single
  continuous ring with no internal seams.
- Q curves bow out to two synthetic wrist corners (perpendicular to
  the palm centerline) for a rounded heel of palm.
- Straight L edges between MCPs along the top - the filter dilation
  rounds the corners visually, no creux.
- Each finger path starts half a stroke inside the palm so the round
  base cap is hidden under the palm fill.
- Whole silhouette shrunk to 65% of the tracked hand size around the
  centroid, with 0.8 group opacity, and a faint MediaPipe skeleton
  overlay (lines + dots) on top.

Update the static fallback silhouettes (HandTrackingFallback) to a
matching curved-path look in a 100x120 viewBox.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/index.css

@@ -1799,7 +1799,8 @@ canvas {
   width: 100vw;
   height: 100vh;
   pointer-events: none;
-  filter: drop-shadow(0 0 8px rgba(56, 189, 248, 0.55));
+  opacity: 0.8;
+  filter: drop-shadow(0 0 4px rgba(96, 165, 250, 0.3));
 }
 
 .hand-tracking-fallback {
@@ -1813,12 +1814,13 @@ canvas {
 
 .hand-tracking-fallback__icon {
   position: absolute;
-  width: 96px;
+  width: 80px;
   height: 96px;
   fill: #67e8f9;
   stroke: #0c4a6e;
-  stroke-width: 2;
+  stroke-width: 3;
   stroke-linejoin: round;
+  stroke-linecap: round;
   filter: drop-shadow(0 0 8px rgba(56, 189, 248, 0.55));
 }