feat: restore direct download link button in share dialog

Add back the missing direct download link button that was removed.
This button allows users to copy a direct download URL for shared files,
which is different from the share page URL.

Changes:
- Add direct download link button with content_paste_go icon
- Import pub API for download URL generation
- Add hasDownloadLink() method to check if file can be directly downloaded
- Add buildDownloadLink() method to generate direct download URLs
- Only show button for single file selections (not directories)

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

frontend/src/components/prompts/Share.vue

@@ -32,6 +32,16 @@
                 <i class="material-icons">content_paste</i>
               </button>
             </td>
+            <td class="small" v-if="hasDownloadLink()">
+              <button
+                class="action copy-clipboard"
+                :aria-label="$t('buttons.copyDownloadLinkToClipboard')"
+                :title="$t('buttons.copyDownloadLinkToClipboard')"
+                @click="copyToClipboard(buildDownloadLink(link))"
+              >
+                <i class="material-icons">content_paste_go</i>
+              </button>
+            </td>
             <td class="small">
               <button
                 class="action"
@@ -132,7 +142,7 @@
 <script>
 import { mapActions, mapState } from "pinia";
 import { useFileStore } from "@/stores/file";
-import { share as api } from "@/api";
+import { share as api, pub as pubApi } from "@/api";
 import dayjs from "dayjs";
 import { useLayoutStore } from "@/stores/layout";
 import { copy } from "@/utils/clipboard";
@@ -247,6 +257,14 @@ export default {
     buildLink(share) {
       return api.getShareURL(share);
     },
+    hasDownloadLink() {
+      return (
+        this.selectedCount === 1 && !this.req.items[this.selected[0]].isDir
+      );
+    },
+    buildDownloadLink(share) {
+      return pubApi.getDownloadURL(share);
+    },
     sort() {
       this.links = this.links.sort((a, b) => {
         if (a.expire === 0) return -1;