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/tagcompleter.h

@@ -0,0 +1,26 @@
+#pragma once
+
+#include <QCompleter>
+#include <QStringList>
+
+class QStringListModel;
+
+// A QCompleter for a comma-separated tags line edit. It completes only the
+// token currently being typed (the text after the last comma) and, on
+// selection, splices the chosen tag back in place of that token — the same
+// per-token behaviour as the web and Android tag fields. Attach with
+// QLineEdit::setCompleter; call setSuggestions() to (re)fill the tag list.
+class TagCompleter : public QCompleter
+{
+public:
+    explicit TagCompleter(QObject *parent = nullptr);
+
+    void setSuggestions(const QStringList &tags);
+
+protected:
+    QStringList splitPath(const QString &path) const override;
+    QString pathFromIndex(const QModelIndex &index) const override;
+
+private:
+    QStringListModel *m_model;
+};