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.cpp

@@ -0,0 +1,92 @@
+#include "managepostwidget.h"
+
+#include <QHBoxLayout>
+#include <QImage>
+#include <QLabel>
+#include <QPixmap>
+#include <QPushButton>
+#include <QVBoxLayout>
+
+namespace {
+constexpr int kThumbSize = 72;
+}
+
+ManagePostWidget::ManagePostWidget(const QString &slug, const QString &caption,
+                                   const QString &date, const QStringList &tags,
+                                   const QString &url, QWidget *parent)
+    : QWidget(parent)
+    , m_slug(slug)
+    , m_caption(caption)
+    , m_date(date)
+    , m_tags(tags)
+    , m_url(url)
+{
+    auto *row = new QHBoxLayout(this);
+    row->setContentsMargins(0, 4, 0, 4);
+
+    m_thumb = new QLabel;
+    m_thumb->setFixedSize(kThumbSize, kThumbSize);
+    m_thumb->setAlignment(Qt::AlignCenter);
+    m_thumb->setFrameShape(QFrame::Box);
+    row->addWidget(m_thumb, 0, Qt::AlignTop);
+
+    auto *fields = new QVBoxLayout;
+    m_captionLabel = new QLabel;
+    m_captionLabel->setTextFormat(Qt::RichText);
+    m_captionLabel->setOpenExternalLinks(true);
+    m_captionLabel->setWordWrap(true);
+    fields->addWidget(m_captionLabel);
+
+    m_metaLabel = new QLabel;
+    m_metaLabel->setWordWrap(true);
+    m_metaLabel->setStyleSheet(QStringLiteral("color:#a89984;"));
+    fields->addWidget(m_metaLabel);
+    row->addLayout(fields, 1);
+
+    m_edit = new QPushButton(QStringLiteral("Edit"));
+    connect(m_edit, &QPushButton::clicked, this, [this]() { emit editRequested(this); });
+    row->addWidget(m_edit, 0, Qt::AlignTop);
+
+    m_delete = new QPushButton(QStringLiteral("Delete"));
+    connect(m_delete, &QPushButton::clicked, this, [this]() { emit deleteRequested(this); });
+    row->addWidget(m_delete, 0, Qt::AlignTop);
+
+    refreshSummary();
+}
+
+void ManagePostWidget::setThumbnail(const QImage &image)
+{
+    if (image.isNull()) {
+        m_thumb->setText(QStringLiteral("?"));
+        return;
+    }
+    m_thumb->setPixmap(QPixmap::fromImage(image).scaled(
+        kThumbSize, kThumbSize, Qt::KeepAspectRatioByExpanding, Qt::SmoothTransformation));
+}
+
+void ManagePostWidget::applyEdit(const QString &caption, const QString &date,
+                                 const QStringList &tags)
+{
+    m_caption = caption;
+    m_date = date;
+    m_tags = tags;
+    refreshSummary();
+}
+
+void ManagePostWidget::setBusy(bool busy)
+{
+    m_edit->setEnabled(!busy);
+    m_delete->setEnabled(!busy);
+}
+
+void ManagePostWidget::refreshSummary()
+{
+    const QString shown = m_caption.isEmpty() ? m_slug : m_caption;
+    m_captionLabel->setText(
+        QStringLiteral("<a href=\"%1\">%2</a>").arg(m_url, shown.toHtmlEscaped()));
+
+    QString meta = m_date;
+    if (!m_tags.isEmpty())
+        meta += QStringLiteral("  ·  ") + m_tags.join(QStringLiteral(", "));
+    m_metaLabel->setText(meta);
+}