gui: category tree, menus, toolbar, splitter — build-order step 3

- CategoryPanel: the left tree — All Downloads / Unfinished / Finished,
  then Categories and Queues populated from category.list / queue.list,
  with per-node task counts. Selecting a node emits a TaskSelection.
- DownloadFilterProxy: QSortFilterProxyModel keyed off that selection.
  Client-side for M1 (the whole list fits); asTaskFilter() exposes the
  equivalent TaskFilter for a server-side download.list once paging lands.
- MainWindow: menu bar (Tasks / Downloads / View / Help) sharing QAction
  objects with the toolbar; QSplitter [panel | table]; Delete with a
  confirm; Resume/Pause All; View menu toggles the panel. Actions
  disabled while offline.
- Counts are computed off a throttled 400 ms timer, not the 4 Hz progress
  path. Fixed a debounce-vs-throttle bug found in the first screenshot:
  restarting the timer on every progress tick meant it never fired and
  the status bar sat at "0 of 0 downloads".
- First-run column widths that fit the content.
- tst_downloadfilterproxy: nodes filter to their own rows, the
  Finished/Unfinished split is correct, and the filter is dynamic (a row
  that finishes leaves the Unfinished node with no re-list). Verified
  against mockd --tasks 400 (screenshot: tree counts 75/84/89/83/69 sum
  to 400, status bar "400 of 400, 21 active").

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

gui/CMakeLists.txt

@@ -22,7 +22,9 @@ add_library(velox-gui-lib STATIC
     src/rpc/RpcConnection.cpp
     src/rpc/RpcClient.cpp
     src/models/DownloadTableModel.cpp
+    src/models/DownloadFilterProxy.cpp
     src/widgets/ProgressDelegate.cpp
+    src/mainwindow/CategoryPanel.cpp
     src/mainwindow/MainWindow.cpp
 )