Add bulk multi-photo upload to the desktop app

Mirror the Android bulk feature in the Qt uploader: choose several photos
at once, each a card with its own caption, date, and upload status, and
post the batch.

- Per-photo cards (PhotoItemWidget) in a scrollable list; retry re-posts
  only the failures.
- One upload per photo with a per-item UUID slug suffix so a batch posted
  in the same second cannot collide.
- Image prep runs off the UI thread via QtConcurrent so the window stays
  responsive.
- A "Simultaneous uploads" setting (1–8, default 3) caps how many photos
  are prepared/uploaded at once; the rest queue and fill freed slots.

desktop/settings.h

@@ -27,6 +27,12 @@ public:
     QString username() const;
     void setUsername(const QString &value);
 
+    // Max photos uploaded at once in a bulk post, clamped to [1, kMaxConcurrency].
+    static constexpr int kMaxConcurrency = 8;
+    static constexpr int kDefaultConcurrency = 3;
+    int concurrency() const;
+    void setConcurrency(int value);
+
     // Keychain access is synchronous here (a local event loop): the app does
     // nothing until the credential is in hand, and there is no background retry
     // worker as on Android that would need it off the UI thread.