gui: finish Add URL -> File Info -> Progress dialog flow

Wires up the three dialogs from build order step 5 (docs/03-gui-spec.md
§§2-3) and the MainWindow slots that were declared but never implemented:

- AddUrlDialog: clipboard prefill is the explicit path docs/06 R2 calls
  for (no passive monitoring, not advertised).
- FileInfoDialog: async download.probe never blocks the UI; ends by
  calling download.add itself (Now / Later / Add to Queue). buildSpec()
  is a pure static so the optional-field-omission logic is unit-testable
  without touching a widget.
- ProgressDialog: non-modal, WA_DeleteOnClose, driven by the taskProgress/
  taskStateChanged signals RpcClient already re-broadcasts; download.get
  seeds state once for a dialog opened mid-transfer. Hosts SegmentBarsWidget
  and SpeedGraphWidget.

MainWindow: openAddUrlDialog/openPropertiesForSelection/showTableContextMenu
now have bodies; category.list/queue.list responses are cached so File Info
can populate its category combo and queue menu without a second round trip.
The row context menu covers what already exists (Resume/Pause/Stop/Delete/
Properties) and deliberately leaves out Open/Open With/Move-Rename/
Redownload/Add to Queue — those need dialogs later build-order steps haven't
reached yet.

util/Format.hpp: pulled the bytes/rate/eta formatting out of MainWindow and
DownloadTableModel once the dialogs wanted the same strings a third time.

Verified end-to-end against a running mockd (category.list/queue.list,
download.probe, download.add, download.get, and live event.task.progress/
event.task.state) under ASan+UBSan: all three dialogs render correctly
against real fixture data and the flow runs clean with no leaks or
sanitizer reports.

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

gui/tests/CMakeLists.txt

@@ -70,6 +70,18 @@ set_tests_properties(gui_speedgraphwidget PROPERTIES
     LABELS "gui"
     ENVIRONMENT "QT_QPA_PLATFORM=offscreen")
 
+# tst_fileinfodialog — FileInfoDialog::buildSpec, the pure DownloadSpec builder.
+#   Red when: an empty optional field starts getting sent instead of omitted, or the
+#   queueId stops being dropped for a startMode other than "queue".
+add_executable(tst_fileinfodialog tst_fileinfodialog.cpp)
+target_compile_features(tst_fileinfodialog PRIVATE cxx_std_23)
+target_compile_options(tst_fileinfodialog PRIVATE -Wall -Wextra -Wpedantic -Werror)
+target_link_libraries(tst_fileinfodialog PRIVATE velox-gui-lib Qt6::Widgets Qt6::Test)
+add_test(NAME gui_fileinfodialog COMMAND tst_fileinfodialog)
+set_tests_properties(gui_fileinfodialog PROPERTIES
+    LABELS "gui"
+    ENVIRONMENT "QT_QPA_PLATFORM=offscreen")
+
 # gui_no_download_logic — CLAUDE.md §3 as an executable check, not a hope.
 #   Red when: a download-logic token (curl, raw pwrite, sqlite, QSqlDatabase) appears
 #   under gui/src. grep exits 0 only when it finds a match, so a hit fails the test.