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/rpc/Protocol.hpp

@@ -0,0 +1,87 @@
+// Shared RPC vocabulary for the GUI client. Lane GUI.
+//
+// The wire is JSON-RPC 2.0 over the Unix socket, newline-delimited (one object per line) —
+// exactly what tools/mockd and the real veloxd speak on the UDS transport. This header
+// carries only the small enums and structs the rpc/ layer passes around; the payload
+// *types* live in libveloxproto and are parsed at the model boundary.
+
+#pragma once
+
+#include <QJsonObject>
+#include <QJsonValue>
+#include <QMetaType>
+#include <QString>
+
+namespace velox::gui::rpc {
+
+/// Where the connection is, for the status-bar dot and the offline banner. Anything other
+/// than Connected means the table is stale and toolbar actions should be disabled.
+enum class ConnectionState {
+    Disconnected,  ///< start() not called yet, or stop() was called
+    Connecting,    ///< TCP/UDS connect in flight
+    Handshaking,   ///< socket up; session.hello / session.subscribe in flight
+    Connected,     ///< handshake done, events flowing
+    Reconnecting,  ///< lost the socket, backing off before the next attempt
+};
+
+/// Bare label for the status bar. Not tr()'d here — callers wrap it.
+inline const char *toString(ConnectionState s) noexcept {
+    switch (s) {
+        case ConnectionState::Disconnected:
+            return "Disconnected";
+        case ConnectionState::Connecting:
+            return "Connecting";
+        case ConnectionState::Handshaking:
+            return "Connecting";
+        case ConnectionState::Connected:
+            return "Connected";
+        case ConnectionState::Reconnecting:
+            return "Reconnecting";
+    }
+    return "Unknown";
+}
+
+/// A JSON-RPC error object, or the absence of one (code == 0).
+struct RpcError {
+    int code = 0;
+    QString message;
+    QJsonValue data;
+
+    bool isError() const noexcept { return code != 0; }
+};
+
+/// The outcome of one call: exactly one of result / error is meaningful.
+struct RpcReply {
+    QJsonValue result;
+    RpcError error;
+
+    bool ok() const noexcept { return !error.isError(); }
+};
+
+// --- method names ---------------------------------------------------------------------
+namespace method {
+inline constexpr auto kSessionHello = "session.hello";
+inline constexpr auto kSessionSubscribe = "session.subscribe";
+inline constexpr auto kDownloadList = "download.list";
+inline constexpr auto kDownloadGet = "download.get";
+inline constexpr auto kDownloadStart = "download.start";
+inline constexpr auto kDownloadPause = "download.pause";
+inline constexpr auto kDownloadResume = "download.resume";
+inline constexpr auto kDownloadCancel = "download.cancel";
+inline constexpr auto kDownloadRemove = "download.remove";
+}  // namespace method
+
+// --- event names ---------------------------------------------------------------------
+namespace event {
+inline constexpr auto kTaskAdded = "event.task.added";
+inline constexpr auto kTaskRemoved = "event.task.removed";
+inline constexpr auto kTaskState = "event.task.state";
+inline constexpr auto kTaskProgress = "event.task.progress";
+inline constexpr auto kSpeedGlobal = "event.speed.global";
+inline constexpr auto kNotify = "event.notify";
+}  // namespace event
+
+}  // namespace velox::gui::rpc
+
+Q_DECLARE_METATYPE(velox::gui::rpc::RpcReply)
+Q_DECLARE_METATYPE(velox::gui::rpc::ConnectionState)