fix(chat): centre the agent-thread terminating dot on the rail (#6059)

The timeline's terminating dot used a single left offset (-17px) at both
breakpoints, but the thread's padding-left differs (22px desktop, 18px
mobile) and the step dots already carry a per-breakpoint offset. The 6px
dot therefore landed 2px right of the 2px rail on desktop and 2px left of
it on mobile, which is the visible kink under an expanded last step.

Derive each offset from the rail's centre instead: the rail sits at
left:5px and is 2px wide, so the dot's left edge belongs at 3px, giving
3px - padding-left per breakpoint.

static/style.css

@@ -9685,11 +9685,11 @@ body.fullwidth-chat .chat-history {
 .agent-thread:not(.has-bottom) .agent-thread-node.open:last-child::after {
   content: '';
   position: absolute;
-  /* -17px (was -15) nudges the terminating dot 2px further left so it
-     sits flush with the thread's left rail when the search node is
-     expanded. This is the "big glow dot at the bottom" the user sees
-     after a web_search step. */
-  left: -17px;
+  /* Centre the 6px dot on the rail: the rail sits at left:5px + 2px wide,
+     so its centre is 6px from the thread's left edge and the dot's left
+     edge belongs at 3px — i.e. 3px - padding-left. Keep this in step with
+     .agent-thread-dot and with the mobile padding override below. */
+  left: -19px;
   bottom: 5px;
   width: 6px;
   height: 6px;
@@ -9958,6 +9958,11 @@ body.fullwidth-chat .chat-history {
     left: -16px;
     top: 10px;
   }
+  /* padding-left drops to 18px here, so the terminating dot needs its own
+     offset (3px - 18px) to stay centred on the rail. */
+  .agent-thread:not(.has-bottom) .agent-thread-node.open:last-child::after {
+    left: -15px;
+  }
 }
 
 /* ===== AGENT TOOL OUTPUT (inside thread nodes) ===== */