gui: RPC client, download table model, and a live main window

First vertical slice of velox-gui, built entirely against tools/mockd
(no daemon dependency):

- rpc/: RpcConnection runs a QLocalSocket on a worker thread with
  newline-delimited JSON-RPC framing, drives the session.hello /
  session.subscribe handshake, and reconnects with exponential backoff
  (250 ms -> 8 s). RpcClient is the main-thread face: marshals calls onto
  the worker, delivers replies as main-thread callbacks, re-emits server
  notifications as typed Qt signals, and issues the one-shot download.list
  on reaching Connected.
- models/DownloadTableModel: QAbstractTableModel over TaskSummary. A
  progress batch is a row patch with a narrow dataChanged over the value
  columns only; beginResetModel() is reserved for the initial load and a
  reconnect resync.
- widgets/ProgressDelegate: in-cell progress bar for the Status column.
- mainwindow/MainWindow: the table, a status-bar connection dot, an
  offline banner instead of a modal, dialog-free pause/resume/stop
  actions, and QSettings column/geometry persistence.
- gui/CMakeLists.txt links velox::proto (never velox::core, ADR 0009) and
  self-guards on the veloxproto target so main keeps configuring if it is
  ever absent again.
- i18n from the first commit: every string via tr(), plus an Arabic .ts
  stub for the RTL check.
- tests/: headless QTest for the model — proves the progress patch is a
  narrow dataChanged and never resets the model.

Verified end-to-end against `mockd --tasks 300`: handshake, initial list,
live progress batches applied to the model, and a clean
Reconnecting -> Connected recovery when mockd is bounced mid-run.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_016Ne28kx4VreeBWZv82Nksd

gui/src/mainwindow/MainWindow.hpp

@@ -0,0 +1,62 @@
+// The main window. Lane GUI.
+//
+// M1 slice: category tree, dialogs and the tray come later. This is the table driven by
+// mockd, a status-bar connection dot, an offline banner instead of a modal, and the three
+// selection actions that need no dialog (pause / resume / cancel).
+
+#pragma once
+
+#include <QMainWindow>
+
+#include "rpc/Protocol.hpp"
+
+class QLabel;
+class QSortFilterProxyModel;
+class QTreeView;
+
+namespace velox::gui {
+
+class DownloadTableModel;
+namespace rpc {
+class RpcClient;
+}  // namespace rpc
+
+class MainWindow : public QMainWindow {
+    Q_OBJECT
+
+  public:
+    explicit MainWindow(rpc::RpcClient *client, QWidget *parent = nullptr);
+    ~MainWindow() override;
+
+  protected:
+    void closeEvent(QCloseEvent *event) override;
+
+  private slots:
+    void onConnectionState(rpc::ConnectionState state);
+    void onSpeedGlobal(const QJsonObject &params);
+    void updateCounts();
+
+    void pauseSelection();
+    void resumeSelection();
+    void cancelSelection();
+
+  private:
+    QStringList selectedTaskIds() const;
+    void actOnSelection(const char *methodName);
+    void restoreLayout();
+    void saveLayout();
+
+    rpc::RpcClient *client_;
+    DownloadTableModel *model_;
+    QSortFilterProxyModel *proxy_;
+    QTreeView *view_;
+
+    QLabel *connDot_;
+    QLabel *connText_;
+    QLabel *countsLabel_;
+    QWidget *offlineBanner_;
+
+    qint64 globalDownBps_ = 0;
+};
+
+}  // namespace velox::gui