[Spoilers] Preserve spoiler files through update and reload failures (#7707)

* Preserve spoiler files and serialize update reloads

* Surface spoiler removal reload failures and allow disabled-state retry

* Address review feedback on spoiler update lifecycle

Only remove the spoiler file when the user unchecks the setting; opening
Settings no longer queues an unrequested reload for a leftover file.
Remove the spoilersUpdatedSuccessfully signal, which had no receiver.

* Fix spoiler settings state after reopen and while disabled

Reopen Settings while an application-owned update is running: the new page
now observes that updater, so the button reads Updating... and re-enables
on completion instead of offering a second update that cancels the first.
Observation does not take ownership: only updaters the page creates are
retired with deleteLater, because the launch check is stack-allocated.

When spoilers are off, the hint now describes what Reload Card Database
actually does, stays readable, and wraps instead of clipping.

---------

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>

tests/CMakeLists.txt

@@ -190,6 +190,15 @@ cockatrice_add_test(
   ${CMAKE_SOURCE_DIR}/cockatrice/src
   ${CMAKE_SOURCE_DIR}/libcockatrice_network
 )
+cockatrice_add_test(
+  NAME
+  spoiler_file_test
+  SOURCES
+  ${CMAKE_SOURCE_DIR}/cockatrice/src/client/network/update/card_spoiler/spoiler_file.cpp
+  spoiler_file_test.cpp
+  INCLUDE_DIRS
+  ${CMAKE_SOURCE_DIR}/cockatrice/src
+)
 # The installer's holder probe is Windows-only, but the wording it produces and the
 # question which programs are the payload's own are plain C++ and are what a stuck user
 # has to act on, so they are tested everywhere.