Add a tags field to the Android and desktop uploaders

Each photo card gains a free-text tags input, sent as a raw
comma-separated `tags` form field on upload. The server normalizes it
(parseTags), so the clients stay dumb — no tag parsing on-device.

- Android: PickedItem.tags + setTags; threaded through UploadQueue →
  UploadWorker → UploadClient's multipart.
- Desktop: a tags QLineEdit per PhotoItemWidget; UploadClient.upload sends
  it.

desktop/uploadclient.h

@@ -29,7 +29,7 @@ public:
     // The filename matters: server.go derives the slug from it, so re-sending
     // the same name overwrites the same post instead of duplicating it.
     void upload(const QString &filePath, const QString &fileName,
-                const QString &caption, const QString &date);
+                const QString &caption, const QString &date, const QString &tags);
 
     // Credential check via GET /upload (same Basic-auth gate the POST uses).
     // /health is not usable: nginx only proxies location /upload.