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/mainwindow.h

@@ -2,6 +2,7 @@
 
 #include <QList>
 #include <QMainWindow>
+#include <QStringList>
 
 #include "settings.h"
 
@@ -13,6 +14,8 @@ class QSpinBox;
 class QStackedWidget;
 class QVBoxLayout;
 class PhotoItemWidget;
+class ManagePostWidget;
+class UploadClient;
 
 class MainWindow : public QMainWindow
 {
@@ -23,9 +26,11 @@ public:
 private:
     QWidget *buildSettingsPage();
     QWidget *buildPostPage();
+    QWidget *buildManagePage();
 
     void showSettings();
     void showPost();
+    void showManage();
 
     void saveAndTest();
     void chooseImages();
@@ -38,6 +43,17 @@ private:
     int inFlightCount() const;           // items currently preparing or uploading
     void refreshPosting();               // updates the Post button + summary from item states
 
+    // Manage page.
+    UploadClient *manageClient();        // lazily-built, lives as long as the window
+    void loadManage();                   // fetches the post list and thumbnails
+    void clearManageItems();
+    void editItem(ManagePostWidget *item);
+    void deleteItem(ManagePostWidget *item);
+
+    // Tag autocomplete.
+    void refreshTags();                  // fetches /api/tags.json
+    void applyTagSuggestions();          // pushes the list onto every post card
+
     Settings m_settings;
 
     QStackedWidget *m_stack = nullptr;
@@ -66,4 +82,16 @@ private:
     QString m_batchUsername;
     QString m_batchPassword;
     QString m_batchCacheDir;
+
+    // Manage page
+    QVBoxLayout *m_manageLayout = nullptr; // holds the ManagePostWidgets
+    QList<ManagePostWidget *> m_manageItems;
+    QLabel *m_manageEmptyHint = nullptr;
+    QLabel *m_manageStatus = nullptr;
+    QPushButton *m_manageRefresh = nullptr;
+    UploadClient *m_manageClient = nullptr; // lazily created, reused for manage ops
+
+    // Known tag names for autocomplete, shared by the post cards and the edit
+    // dialog. Best-effort: empty until /api/tags.json arrives.
+    QStringList m_tagSuggestions;
 };