feat: configurable send shortcut (Enter vs Ctrl+Enter)

Add toggle in chat input to switch between Enter-to-send (default)
and Ctrl+Enter-to-send modes. Preference persists in localStorage.
Keyboard shortcuts modal reflects the current setting.

src/hooks/useSendShortcut.ts

@@ -0,0 +1,21 @@
+import { useState, useCallback } from 'react';
+
+const STORAGE_KEY = 'pinchchat-send-on-enter';
+
+/** Hook to manage the send shortcut preference (Enter vs Ctrl+Enter). */
+export function useSendShortcut() {
+  const [sendOnEnter, setSendOnEnter] = useState(() => {
+    const stored = localStorage.getItem(STORAGE_KEY);
+    return stored === null ? true : stored === 'true';
+  });
+
+  const toggle = useCallback(() => {
+    setSendOnEnter(prev => {
+      const next = !prev;
+      localStorage.setItem(STORAGE_KEY, String(next));
+      return next;
+    });
+  }, []);
+
+  return { sendOnEnter, toggle };
+}