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/tests/CMakeLists.txt

@@ -22,6 +22,16 @@ target_link_libraries(tst_downloadtablemodel PRIVATE Qt6::Core Qt6::Test)
 add_test(NAME gui_downloadtablemodel COMMAND tst_downloadtablemodel)
 set_tests_properties(gui_downloadtablemodel PROPERTIES LABELS "gui")
 
+# tst_downloadfilterproxy — the category-tree filter.
+#   Red when: a node stops filtering to its own rows, the Finished/Unfinished split
+#   misclassifies a state, or the filter stops being dynamic.
+add_executable(tst_downloadfilterproxy tst_downloadfilterproxy.cpp)
+target_compile_features(tst_downloadfilterproxy PRIVATE cxx_std_23)
+target_compile_options(tst_downloadfilterproxy PRIVATE -Wall -Wextra -Wpedantic -Werror)
+target_link_libraries(tst_downloadfilterproxy PRIVATE velox-gui-lib Qt6::Test)
+add_test(NAME gui_downloadfilterproxy COMMAND tst_downloadfilterproxy)
+set_tests_properties(gui_downloadfilterproxy PROPERTIES LABELS "gui")
+
 # tst_rtl — the real RTL check the GUI DoD asks for (the .ts stub alone verifies nothing).
 #   Red when: the main window stops propagating Qt::RightToLeft to its children, or the
 #   offline-banner layout is pinned so it does not mirror under RTL.