fix(race-story): sync chapter highlight and empty-state E2E (#68)

Clamp active-chapter detection to the same scrub bounds as chapter clicks,
keep an explicit selection on click for out-of-window timestamps, point E2E
at the empty-state card, and move Race Story empty-state styles out of app.css.

Co-authored-by: Cursor <cursoragent@cursor.com>

frontend/src/styles/app.css

@@ -520,17 +520,6 @@ a { color: inherit; text-decoration: none; }
   color: var(--text-3);
 }
 
-.empty-state-icon {
-  display: block;
-  margin: 0 auto var(--s3);
-  color: var(--text-3);
-}
-
-.race-story-empty-card {
-  padding: var(--s6) var(--s5);
-  margin: 0;
-}
-
 .loading-state {
   padding: var(--s7) 0;
   text-align: center;

frontend/src/styles/chapters.css

@@ -196,6 +196,7 @@
 }
 
 .chapter-strip-empty-card .empty-state-icon {
+  display: block;
   margin: 0 auto var(--s3);
   color: var(--text-3);
 }

frontend/src/styles/race-story.css

@@ -1,3 +1,14 @@
+.race-story-empty-card {
+  padding: var(--s6) var(--s5);
+  margin: 0;
+}
+
+.race-story-empty-card .empty-state-icon {
+  display: block;
+  margin: 0 auto var(--s3);
+  color: var(--text-3);
+}
+
 .rs-replay-shell {
   display: grid;
   grid-template-columns: 1fr;

frontend/src/styles/replay-map.css

@@ -85,6 +85,7 @@
 }
 
 .replay-map-empty-card .empty-state-icon {
+  display: block;
   margin: 0 auto var(--s3);
   color: var(--text-3);
 }