ci(app): Flutter CI-джоба + сборка Linux/Windows

analyze+test и flutter build linux/windows в отдельных джобах по образцу
backend. Линуксовая сборка требует libsecret-1-dev/pkg-config (apt) и
подавления -Werror=deprecated-literal-operator в CMake — flutter_secure_storage_linux
вендорит старый nlohmann/json.hpp, который на новых компиляторах иначе
не собирается.

app/linux/CMakeLists.txt

@@ -70,6 +70,16 @@ set_target_properties(${BINARY_NAME}
 )
 
 
+# flutter_secure_storage_linux vendors an old single-header nlohmann/json.hpp
+# that trips modern Clang's -Werror=deprecated-literal-operator (whitespace
+# before a literal-operator suffix, e.g. `operator"" _json`). We don't control
+# that vendored header, so silence just this warning for everything added
+# below (must come before add_subdirectory pulls the plugins in — Clang picks
+# it up because add_compile_options affects this directory scope and every
+# add_subdirectory call after it, which is how generated_plugins.cmake adds
+# the plugin targets).
+add_compile_options(-Wno-deprecated-literal-operator)
+
 # Generated plugin build rules, which manage building the plugins and adding
 # them to the application.
 include(flutter/generated_plugins.cmake)