fix(webapp): keep the hero's heading reachable when it outgrows the panel

justify-center on a scrolling column overflows equally in both directions, and
nothing can scroll back past the origin, so at the docked panel's narrowest the
heading and composer were unreachable. The child centres with m-auto, which gives
its space up once there is none to spare.

Author: kathiekiwi

apps/webapp/app/components/dashboard-agent/DashboardAgentHero.tsx

@@ -19,9 +19,11 @@ export function DashboardAgentHero({
   dismissedIds?: string[];
   composer?: React.ReactNode;
 }) {
+  // Centred by the child's `m-auto`, not by `justify-center`: auto margins give up their space
+  // once the content outgrows the panel, so the heading stays scrollable to.
   return (
-    <div className="flex min-h-0 flex-1 flex-col items-center justify-center overflow-y-auto px-4 py-6 scrollbar-thin scrollbar-track-transparent scrollbar-thumb-surface-control">
-      <div className="flex w-full max-w-2xl flex-col items-center gap-5">
+    <div className="flex min-h-0 flex-1 flex-col overflow-y-auto px-4 py-6 scrollbar-thin scrollbar-track-transparent scrollbar-thumb-surface-control">
+      <div className="m-auto flex w-full max-w-2xl flex-col items-center gap-5">
         <div className="flex flex-col items-center gap-1.5 text-center">
           <Header1 className="flex items-center gap-2">
             <AgentMonoLogo size={22} decorative />

apps/webapp/app/components/dashboard-agent/chat-layout.test.ts

@@ -101,4 +101,27 @@ describe("chat-layout enforcement", () => {
       expect(source).not.toMatch(/bg-indigo-\d/);
     });
   });
+
+  /**
+   * Structural guard, not behavioural proof: the webapp has no DOM test environment, so nothing
+   * here lays the panel out or scrolls it. It asserts the class combination that loses the top
+   * of an overflowing column is absent — `justify-center` on a scroll container centres by
+   * distributing free space, and negative free space overflows past the scroll origin, where a
+   * child's `m-auto` collapses to zero instead.
+   */
+  describe("scrolling panes", () => {
+    const SCROLLERS = ["DashboardAgentHero.tsx"];
+
+    it.each(SCROLLERS)("%s centres an overflowing column with auto margins", (file) => {
+      const source = read(file);
+      const scrollLines = source
+        .split("\n")
+        .filter((line) => line.includes("overflow-y-auto") || line.includes("overflow-auto"));
+      expect(scrollLines.length).toBeGreaterThan(0);
+      for (const line of scrollLines) {
+        expect(line, line.trim()).not.toMatch(/\bjustify-center\b/);
+      }
+      expect(source).toMatch(/\bm-auto\b/);
+    });
+  });
 });