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/style.css

@@ -199,6 +199,7 @@ button:hover, select:hover {
 /* Input bar */
 #input {
   display: flex;
+  align-items: flex-end;   /* arrow stays at the bottom as the textarea grows */
   border-top: 1px solid var(--border);
   background: var(--bg-elev);
   padding: 10px;
@@ -208,9 +209,14 @@ button:hover, select:hover {
   background: var(--bg-line);
   color: var(--text-main);
   border: 1px solid var(--border);
-  border-radius: 8px;
+  border-radius: 16px;
   padding: 9px 12px;
   font-family: var(--font-console);
+  font-size: 0.95rem;
+  line-height: 1.4;
+  resize: none;             /* grown programmatically, not by the drag handle */
+  max-height: 140px;
+  overflow-y: auto;
   transition: border-color .15s, box-shadow .15s;
 }
 #userInput::placeholder { color: var(--text-fade); }
@@ -221,6 +227,16 @@ button:hover, select:hover {
 }
 #sendBtn {
   margin-left: 8px;
+  flex: none;
+  width: 38px;
+  height: 38px;
+  padding: 0;
+  border-radius: 50%;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  font-size: 1.25rem;
+  line-height: 1;
   background: var(--accent);
   color: #0a0a0a;
   border-color: var(--accent);
@@ -936,12 +952,14 @@ select:hover {
 
   #userInput {
     font-size: 16px; /* Prevents zoom on iOS */
-    padding: 12px;
+    padding: 11px 14px;
   }
 
   #sendBtn {
-    padding: 12px 16px;
-    font-size: 1rem;
+    width: 44px;     /* comfortable touch target */
+    height: 44px;
+    padding: 0;
+    font-size: 1.35rem;
   }
 
   /* Modal - full width on mobile */
@@ -1040,12 +1058,14 @@ select:hover {
 
   #userInput {
     font-size: 16px;
-    padding: 10px;
+    padding: 10px 13px;
   }
 
   #sendBtn {
-    padding: 10px 14px;
-    font-size: 0.95rem;
+    width: 42px;
+    height: 42px;
+    padding: 0;
+    font-size: 1.3rem;
   }
 
   .modal-header h3 {