pref: pin save button to the bottom of the page in theme preview (#6449)

#### What type of PR is this?

/kind improvement
/area ui
/milestone 2.19.x

#### What this PR does / why we need it:

将主题预览中的保存功能固定在底部，方便当主题设置项过多时进行保存。

为了不影响保存功能，因此此 PR 移除了本来在左下角悬浮的选择主题按钮。选择主题可以直接在右上角进行点击。

#### Which issue(s) this PR fixes:

Fixes #6448 

#### Does this PR introduce a user-facing change?
```release-note
在主题预览界面中将保存按钮固定在底部
```

ui/console-src/modules/interface/themes/components/preview/ThemePreviewModal.vue

@@ -9,7 +9,6 @@ import type {
 } from "@halo-dev/api-client";
 import { consoleApiClient } from "@halo-dev/api-client";
 import {
-  IconArrowLeft,
   IconComputer,
   IconLink,
   IconPalette,
@@ -29,6 +28,7 @@ import { storeToRefs } from "pinia";
 import { computed, markRaw, onMounted, ref, toRaw } from "vue";
 import { useI18n } from "vue-i18n";
 import ThemePreviewListItem from "./ThemePreviewListItem.vue";
+import StickyBlock from "@/components/sticky-block/StickyBlock.vue";
 
 const props = withDefaults(
   defineProps<{
@@ -333,8 +333,11 @@ const iframeClasses = computed(() => {
                     />
                   </FormKit>
                 </div>
-                <div v-permission="['system:themes:manage']" class="pt-5">
+                <StickyBlock
-                  <div class="flex justify-start">
+                  v-permission="['system:themes:manage']"
+                  class="-mx-4 -mb-4 -mr-3 rounded-b-base rounded-t-lg bg-white p-4 pt-5"
+                  position="bottom"
+                >
                   <VButton
                     :loading="saving"
                     type="secondary"
@@ -346,8 +349,7 @@ const iframeClasses = computed(() => {
                   >
                     {{ $t("core.common.buttons.save") }}
                   </VButton>
-                  </div>
+                </StickyBlock>
-                </div>
               </div>
             </div>
           </transition>
@@ -383,25 +385,6 @@ const iframeClasses = computed(() => {
               </li>
             </ul>
           </transition>
-          <transition
-            enter-active-class="transform transition ease-in-out duration-300"
-            enter-from-class="translate-y-full"
-            enter-to-class="translate-y-0"
-            leave-active-class="transform transition ease-in-out duration-300"
-            leave-from-class="translate-y-0"
-            leave-to-class="translate-y-full"
-          >
-            <div v-if="settingsVisible" class="fixed bottom-2 left-2">
-              <VButton
-                size="md"
-                circle
-                type="primary"
-                @click="handleOpenThemes"
-              >
-                <IconArrowLeft />
-              </VButton>
-            </div>
-          </transition>
         </OverlayScrollbarsComponent>
       </transition>
       <div