Update icon beside the search box after the update dialog is dismissed

Shown as soon as an update is offered, so Later, a backdrop tap or a failed
download all leave a one-tap way to update; it targets the newest build and
disappears while updating. The search form and box now shrink so the icon
never pushes Search off a phone screen.

frontend/index.html

@@ -100,6 +100,10 @@
               placeholder="Search YouTube…  (no login, no ads)"
               autocomplete="off"
             />
+            <!-- Shown after "Later" on the update dialog: one tap updates -->
+            <button id="updateIconBtn" type="button" class="update-icon-btn hidden" title="Update available — tap to update" aria-label="Update available — tap to update">
+              <svg viewBox="0 0 24 24" aria-hidden="true"><path d="M20 12a8 8 0 1 1-2.34-5.66M20 4v4.5h-4.5"/><path d="M12 8v5M9.5 11l2.5 2.5 2.5-2.5"/></svg>
+            </button>
             <button type="submit" class="search-btn">Search</button>
             <div id="searchRecent" class="search-recent hidden" role="listbox" aria-label="Recent searches"></div>
           </form>