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/managepostwidget.h

@@ -0,0 +1,48 @@
+#pragma once
+
+#include <QString>
+#include <QStringList>
+#include <QWidget>
+
+class QImage;
+class QLabel;
+class QPushButton;
+
+// One published post on the Manage page: thumbnail, caption/date/tags summary,
+// and Edit / Delete buttons. Mirrors a row of the web /manage list and the
+// Android manage screen.
+class ManagePostWidget : public QWidget
+{
+    Q_OBJECT
+public:
+    ManagePostWidget(const QString &slug, const QString &caption, const QString &date,
+                     const QStringList &tags, const QString &url, QWidget *parent = nullptr);
+
+    QString slug() const { return m_slug; }
+    QString caption() const { return m_caption; }
+    QString date() const { return m_date; }
+    QStringList tags() const { return m_tags; }
+
+    void setThumbnail(const QImage &image);
+    void applyEdit(const QString &caption, const QString &date, const QStringList &tags);
+    void setBusy(bool busy); // a save/delete is in flight
+
+signals:
+    void editRequested(ManagePostWidget *self);
+    void deleteRequested(ManagePostWidget *self);
+
+private:
+    void refreshSummary();
+
+    QString m_slug;
+    QString m_caption;
+    QString m_date;
+    QStringList m_tags;
+    QString m_url;
+
+    QLabel *m_thumb = nullptr;
+    QLabel *m_captionLabel = nullptr;
+    QLabel *m_metaLabel = nullptr;
+    QPushButton *m_edit = nullptr;
+    QPushButton *m_delete = nullptr;
+};