gui: add SegmentBarsWidget and SpeedGraphWidget

Per-connection progress bars and the 60 s rolling speed graph the progress
dialog needs (docs/03-gui-spec.md §3). Both reuse row/widget state across
ticks instead of rebuilding, matching the discipline DownloadTableModel
already uses for progress patches.

SpeedGraphWidget keeps a fixed ring buffer and one reused QPainterPath —
no allocation in paintEvent or addSample. Fixed a real bug found while
writing tst_speedgraphwidget: the elapsed timer was started in the
constructor, so the very first sample after construction would silently
wait up to 1 s to be recorded instead of landing immediately.

Tested against mockd (both offline via QTest/offscreen, and manually
against a running mockd instance through ProgressDialog once that lands).

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

gui/CMakeLists.txt

@@ -24,6 +24,8 @@ add_library(velox-gui-lib STATIC
     src/models/DownloadTableModel.cpp
     src/models/DownloadFilterProxy.cpp
     src/widgets/ProgressDelegate.cpp
+    src/widgets/SegmentBarsWidget.cpp
+    src/widgets/SpeedGraphWidget.cpp
     src/mainwindow/CategoryPanel.cpp
     src/mainwindow/MainWindow.cpp
 )