feat: GripAnalysisView — car grip envelope visualization

- New GripAnalysisView with 4 scatter charts:
  1. Grip Envelope (peak G vs speed: lateral/accel/brake)
  2. Grip Circle (longitudinal vs lateral G scatter)
  3. Cornering Grip vs Speed
  4. Steering Response (steer angle vs lateral G)
- Summary stats: max lateral/accel/brake/combined G, peak lateral speed
- GripAnalysis types + type guard
- car_grip use case in SessionDetailView (quick stats + view button)
- Route /analysis/grip/:id
- Global .grid-4 CSS utility

src/assets/main.css

@@ -196,8 +196,12 @@ input, select, textarea {
   grid-template-columns: repeat(3, 1fr);
 }
 
+.grid-4 {
+  grid-template-columns: repeat(4, 1fr);
+}
+
 @media (max-width: 768px) {
-  .grid-2, .grid-3 {
+  .grid-2, .grid-3, .grid-4 {
     grid-template-columns: 1fr;
   }
 }