Clear the composer draft when entering the New Chat / welcome state (#1408)

Clicking "New chat" (the brand/welcome navigation path) left the previous
session's unsent draft in the composer (issue #1343). The direct model-picker
path (createDirectChat) already cleared it, but the welcome path did not.

Clear `#message` in chatRenderer.showWelcomeScreen() — the shared entry point
for that state — resetting its autosized height and dispatching an `input` event
so the send button / autosize listeners update. Switching between existing
sessions loads them directly and does not call showWelcomeScreen, so genuine
drafts are not erased.

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

static/js/chatRenderer.js

@@ -1217,6 +1217,17 @@ export function showWelcomeScreen() {
   const cc = document.getElementById('chat-container');
   if (ws) ws.classList.remove('hidden');
   if (cc) cc.classList.add('welcome-active');
+  // Entering the New Chat / welcome state: discard any stale draft left in the
+  // composer from the previous session so the input starts empty (issue #1343).
+  // Switching between existing sessions loads them directly and does NOT call
+  // this, so genuine drafts are not erased. Reset the autosized height and fire
+  // an `input` event so the send button + autosize listeners update.
+  const _msg = document.getElementById('message');
+  if (_msg) {
+    _msg.value = '';
+    _msg.style.height = '';
+    _msg.dispatchEvent(new Event('input', { bubbles: true }));
+  }
   // Re-trigger the L→R clip-wipe reveal on the welcome name each time the
   // welcome screen is shown (new session, deleted last session, etc.) — without
   // this, the CSS animation only fires on initial DOM insertion.