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.cpp

@@ -0,0 +1,42 @@
+#include "tagcompleter.h"
+
+#include <QLineEdit>
+#include <QStringListModel>
+
+TagCompleter::TagCompleter(QObject *parent)
+    : QCompleter(parent)
+    , m_model(new QStringListModel(this))
+{
+    setModel(m_model);
+    setCaseSensitivity(Qt::CaseInsensitive);
+    setFilterMode(Qt::MatchContains); // match anywhere in the tag, like Android
+    setCompletionMode(QCompleter::PopupCompletion);
+}
+
+void TagCompleter::setSuggestions(const QStringList &tags)
+{
+    m_model->setStringList(tags);
+}
+
+// Complete against the token after the last comma, ignoring the tags already
+// entered before it.
+QStringList TagCompleter::splitPath(const QString &path) const
+{
+    return QStringList(path.section(QLatin1Char(','), -1).trimmed());
+}
+
+// Rebuild the full field value: everything up to and including the last comma,
+// then the chosen tag and a trailing ", " ready for the next one.
+QString TagCompleter::pathFromIndex(const QModelIndex &index) const
+{
+    const QString completion = QCompleter::pathFromIndex(index);
+    const auto *edit = qobject_cast<const QLineEdit *>(widget());
+    if (!edit)
+        return completion;
+
+    const QString text = edit->text();
+    const int lastComma = text.lastIndexOf(QLatin1Char(','));
+    const QString head =
+        lastComma >= 0 ? text.left(lastComma + 1) + QLatin1Char(' ') : QString();
+    return head + completion + QStringLiteral(", ");
+}