Improve search input UX on mobile

 - Set an `action` on the form so that the virtual keyboard shows a
   submit button, which makes it easier to trigger a search

 - Use a larger font size for readability and to prevent iOS from
   zooming into the field on desktop websites

src/sidebar/components/search-input.js

@@ -41,7 +41,12 @@ function SearchInput({ alwaysExpanded, query, onSearch }) {
   }
 
   return (
-    <form className="search-input__form" name="searchForm" onSubmit={onSubmit}>
+    <form
+      action="#"
+      className="search-input__form"
+      name="searchForm"
+      onSubmit={onSubmit}
+    >
       <input
         className={classnames('search-input__input', {
           'is-expanded': alwaysExpanded || query,

src/styles/sidebar/components/search-input.scss

@@ -45,3 +45,9 @@
     padding-left: 6px;
   }
 }
+
+@media (pointer: coarse) {
+  .search-input__input {
+    font-size: $touch-input-font-size;
+  }
+}