feat: create ExpandButton component with click/double-click logic

packages/ui/src/components/expand-button.tsx

@@ -0,0 +1,72 @@
+import { createSignal, Show } from "solid-js"
+import { Maximize2, Minimize2 } from "lucide-solid"
+
+interface ExpandButtonProps {
+  expandState: () => "normal" | "fifty" | "eighty"
+  onToggleExpand: (nextState: "normal" | "fifty" | "eighty") => void
+}
+
+export default function ExpandButton(props: ExpandButtonProps) {
+  const [clickTime, setClickTime] = createSignal<number>(0)
+  const DOUBLE_CLICK_THRESHOLD = 300
+
+  function handleClick() {
+    const now = Date.now()
+    const lastClick = clickTime()
+    const isDoubleClick = now - lastClick < DOUBLE_CLICK_THRESHOLD
+
+    setClickTime(now)
+
+    const current = props.expandState()
+
+    if (isDoubleClick) {
+      // Double click behavior
+      if (current === "normal") {
+        props.onToggleExpand("fifty")
+      } else if (current === "fifty") {
+        props.onToggleExpand("eighty")
+      } else {
+        props.onToggleExpand("normal")
+      }
+    } else {
+      // Single click behavior
+      if (current === "normal") {
+        props.onToggleExpand("fifty")
+      } else {
+        props.onToggleExpand("normal")
+      }
+    }
+
+    // Reset click timer after threshold
+    setTimeout(() => setClickTime(0), DOUBLE_CLICK_THRESHOLD)
+  }
+
+  const getTooltip = () => {
+    const current = props.expandState()
+    if (current === "normal") {
+      return "Click to expand (50%) • Double-click to expand further (80%)"
+    } else if (current === "fifty") {
+      return "Double-click to expand to 80% • Click to minimize"
+    } else {
+      return "Click to minimize • Double-click to expand to 50%"
+    }
+  }
+
+  return (
+    <button
+      type="button"
+      class="prompt-expand-button"
+      onClick={handleClick}
+      disabled={false}
+      aria-label="Toggle chat input height"
+      title={getTooltip()}
+    >
+      <Show
+        when={props.expandState() === "normal"}
+        fallback={<Minimize2 class="h-5 w-5" aria-hidden="true" />}
+      >
+        <Maximize2 class="h-5 w-5" aria-hidden="true" />
+      </Show>
+    </button>
+  )
+}