gui: finish Add URL -> File Info -> Progress dialog flow

Wires up the three dialogs from build order step 5 (docs/03-gui-spec.md
§§2-3) and the MainWindow slots that were declared but never implemented:

- AddUrlDialog: clipboard prefill is the explicit path docs/06 R2 calls
  for (no passive monitoring, not advertised).
- FileInfoDialog: async download.probe never blocks the UI; ends by
  calling download.add itself (Now / Later / Add to Queue). buildSpec()
  is a pure static so the optional-field-omission logic is unit-testable
  without touching a widget.
- ProgressDialog: non-modal, WA_DeleteOnClose, driven by the taskProgress/
  taskStateChanged signals RpcClient already re-broadcasts; download.get
  seeds state once for a dialog opened mid-transfer. Hosts SegmentBarsWidget
  and SpeedGraphWidget.

MainWindow: openAddUrlDialog/openPropertiesForSelection/showTableContextMenu
now have bodies; category.list/queue.list responses are cached so File Info
can populate its category combo and queue menu without a second round trip.
The row context menu covers what already exists (Resume/Pause/Stop/Delete/
Properties) and deliberately leaves out Open/Open With/Move-Rename/
Redownload/Add to Queue — those need dialogs later build-order steps haven't
reached yet.

util/Format.hpp: pulled the bytes/rate/eta formatting out of MainWindow and
DownloadTableModel once the dialogs wanted the same strings a third time.

Verified end-to-end against a running mockd (category.list/queue.list,
download.probe, download.add, download.get, and live event.task.progress/
event.task.state) under ASan+UBSan: all three dialogs render correctly
against real fixture data and the flow runs clean with no leaks or
sanitizer reports.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01NSCdCWFXBTSBBK3MzWtJiC

gui/src/util/Format.hpp

@@ -0,0 +1,43 @@
+// Shared human-readable formatting. Lane GUI.
+//
+// Pulled out once three places (the table model, the main window status bar, the
+// progress dialog) all wanted the same "bytes / rate / eta" strings.
+
+#pragma once
+
+#include <QCoreApplication>
+#include <QLocale>
+#include <QString>
+
+namespace velox::gui::fmt {
+
+inline QString bytes(qint64 n) {
+    if (n < 0) {
+        return QStringLiteral("—");
+    }
+    return QLocale().formattedDataSize(n, 2, QLocale::DataSizeIecFormat);
+}
+
+inline QString rate(qint64 bytesPerSec) {
+    if (bytesPerSec <= 0) {
+        return QCoreApplication::translate("velox::gui::fmt", "0 B/s");
+    }
+    return QCoreApplication::translate("velox::gui::fmt", "%1/s")
+        .arg(QLocale().formattedDataSize(bytesPerSec, 1, QLocale::DataSizeIecFormat));
+}
+
+/// HH:MM:SS, or empty for a negative (unknown) eta.
+inline QString eta(qint64 secs) {
+    if (secs < 0) {
+        return QString();
+    }
+    const qint64 h = secs / 3600;
+    const qint64 m = (secs % 3600) / 60;
+    const qint64 s = secs % 60;
+    return QStringLiteral("%1:%2:%3")
+        .arg(h, 2, 10, QLatin1Char('0'))
+        .arg(m, 2, 10, QLatin1Char('0'))
+        .arg(s, 2, 10, QLatin1Char('0'));
+}
+
+}  // namespace velox::gui::fmt