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/uploadclient.h

@@ -1,10 +1,23 @@
 #pragma once
 
+#include <QImage>
+#include <QList>
 #include <QObject>
 #include <QString>
+#include <QStringList>
 
 class QNetworkAccessManager;
 
+// One published post, as returned by GET /api/photos.
+struct RemotePhoto {
+    QString slug;
+    QString caption;
+    QString date; // yyyy-MM-dd
+    QStringList tags;
+    QString thumb; // site-relative, e.g. "/thumbs/foo.jpg"
+    QString url;   // site-relative, e.g. "/photo/foo/"
+};
+
 // Talks to the gallery's POST /upload endpoint, which sits behind nginx HTTP
 // Basic auth. The credentials go out preemptively in the Authorization header
 // on every request (never via QNetworkAccessManager's authenticationRequired
@@ -35,12 +48,29 @@ public:
     // /health is not usable: nginx only proxies location /upload.
     void testConnection();
 
+    // Manage API, mirroring the web /manage and the Android app. The desktop
+    // app is a non-browser client: it sends no Origin/Sec-Fetch-Site header, so
+    // the state-changing posts pass the server's same-origin (CSRF) check.
+    void listPhotos();                       // GET /api/photos
+    void fetchTags();                        // GET /api/tags.json (unguarded)
+    void fetchThumbnail(const QString &slug); // GET /api/thumb/<slug> (unguarded)
+    void editPost(const QString &slug, const QString &caption,
+                  const QString &date, const QString &tags); // POST /api/edit
+    void deletePost(const QString &slug);    // POST /api/delete (moves to trash)
+
 signals:
     void uploadFinished(UploadClient::Result result, const QString &slug,
                         const QString &url, const QString &message);
     // errorMessage is empty on success.
     void testFinished(const QString &errorMessage);
 
+    // Manage signals. The error strings are empty on success.
+    void photosListed(const QList<RemotePhoto> &photos, const QString &error);
+    void tagsFetched(const QStringList &tags);
+    void thumbnailReady(const QString &slug, const QImage &image);
+    void editFinished(const QString &slug, const QString &error);
+    void deleteFinished(const QString &slug, const QString &error);
+
 private:
     QByteArray authHeader() const;