feat(web): multiline composer — Enter adds a newline, arrow sends

Stops fat-fingered early sends. The single-line input is now an auto-growing
textarea (Claude-app style): Enter inserts a newline and expands the box (up to a
cap, then it scrolls); you tap the ↑ arrow to send. ⌘/Ctrl+Enter still sends from
a hardware keyboard. Send button is now a round arrow; box resets to one line
after sending. Mobile button is a 42-44px touch target.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lyra/web/static/index.html

@@ -116,8 +116,8 @@
 
     <!-- Input box -->
     <div id="input">
-      <input id="userInput" type="text" placeholder="Type a message..." autofocus />
-      <button id="sendBtn">Send</button>
+      <textarea id="userInput" rows="1" placeholder="Type a message…" autofocus></textarea>
+      <button id="sendBtn" aria-label="Send" title="Send (or ⌘/Ctrl+Enter)">↑</button>
     </div>
 
     <!-- Bottom tab bar (mobile only; hides while the keyboard is open) -->
@@ -287,6 +287,8 @@
       if (!msg) return;
       inputEl.value = "";
 
+      autoGrow(inputEl);  // collapse the box back to one line after clearing
+
       addMessage("user", msg);
       history.push({ role: "user", content: msg });
 	  await saveSession(); // ✅ persist both user + assistant messages
@@ -548,6 +550,13 @@
 	  return b;
 	}
 
+	// Grow the input textarea to fit its content (up to a cap, then it scrolls).
+	function autoGrow(el) {
+	  if (!el) return;
+	  el.style.height = "auto";
+	  el.style.height = Math.min(el.scrollHeight, 140) + "px";
+	}
+
 	function addMessage(role, text, autoScroll = true) {
 	  const messagesEl = document.getElementById("messages");
 
@@ -1004,11 +1013,15 @@
       checkHealth();
       setInterval(checkHealth, 10000);
 
-      // Input events
+      // Input events. Enter inserts a newline and grows the box (like the Claude
+      // app) — you tap the arrow to send. ⌘/Ctrl+Enter sends from the keyboard.
       document.getElementById("sendBtn").addEventListener("click", sendMessage);
-      document.getElementById("userInput").addEventListener("keypress", e => {
-        if (e.key === "Enter") sendMessage();
+      const inputBox = document.getElementById("userInput");
+      inputBox.addEventListener("input", () => autoGrow(inputBox));
+      inputBox.addEventListener("keydown", e => {
+        if (e.key === "Enter" && (e.metaKey || e.ctrlKey)) { e.preventDefault(); sendMessage(); }
       });
+      autoGrow(inputBox);
 
       // ========== THINKING STREAM INTEGRATION ==========
       const thinkingPanel = document.getElementById("thinkingPanel");