Qt desktop: add manage (edit/delete) and tag autocomplete

Brings the Qt uploader to parity with the web /manage page and the Android
app. Adds a stacked Manage page (reached from the post page) that lists
posts via GET /api/photos with thumbnails from GET /api/thumb/<slug>, edits
caption/date/tags in a QDialog via POST /api/edit, and deletes to trash via
POST /api/delete with a QMessageBox confirm. As a non-browser client it sends
no Origin and so passes the server's same-origin CSRF check.

Tag autocomplete now backs every tags field (post cards and the edit dialog)
via a per-token TagCompleter (QCompleter subclass, MatchContains) fed from
the unguarded GET /api/tags.json.

New files: managepostwidget.{h,cpp}, tagcompleter.{h,cpp}; network methods
and a RemotePhoto struct added to uploadclient. Built and linked clean; not
yet runtime-tested on a real display.

desktop/photoitemwidget.h

@@ -7,6 +7,7 @@ class QDateEdit;
 class QLabel;
 class QLineEdit;
 class QPushButton;
+class TagCompleter;
 
 // One photo in the batch: thumbnail, caption, date, and its own upload status.
 // A single-photo post is just a batch of one, so the window runs one code path.
@@ -26,6 +27,9 @@ public:
 
     void setStatus(Status status, const QString &message = {}, const QString &url = {});
 
+    // Fills the tags field's autocomplete with the known tag names.
+    void setTagSuggestions(const QStringList &tags);
+
 signals:
     void removeRequested(PhotoItemWidget *self);
 
@@ -39,4 +43,5 @@ private:
     QLineEdit *m_tags = nullptr;
     QLabel *m_statusLabel = nullptr;
     QPushButton *m_remove = nullptr;
+    TagCompleter *m_tagCompleter = nullptr;
 };