feat(gui): drive everything from settings, add tray and in-app download

The GUI no longer needs command line arguments: on first run it opens a
settings dialog that lists the registry, downloads models in-app (mirror
base, HTTP proxy, resume), and picks the active model. The caption window
gains a tray icon (show/hide, settings, quit); its close button hides to
the tray so the window can always be brought back, and the language pill
opens the settings. Appearance (font, opacity, line counts, position) is
persisted. CLI flags remain for automated testing and override settings.

GUI 不再依赖命令行参数：首次启动打开设置对话框，可在应用内下载模型
（下载源/代理/断点续传）、选择当前模型与识别语言；字幕窗增加托盘（显示
隐藏/设置/退出），关闭按钮改为隐藏到托盘以免关掉后无法再打开，语言标签
点击进设置；字号/透明度/行数/位置都会持久化。命令行参数保留作自动化测试。

CMakeLists.txt

@@ -28,7 +28,7 @@ foreach(_p
     endif()
 endforeach()
 
-find_package(Qt6 REQUIRED COMPONENTS Core Widgets)
+find_package(Qt6 REQUIRED COMPONENTS Core Widgets Network)
 find_package(PkgConfig REQUIRED)
 find_package(Threads REQUIRED)
 
@@ -112,9 +112,10 @@ target_link_libraries(livecaption_core PUBLIC
 # ---------------------------------------------------------------------------
 add_library(livecaption_models STATIC
     src/models/modelstore.cpp
+    src/models/modeldownloader.cpp
 )
 target_include_directories(livecaption_models PUBLIC src/models)
-target_link_libraries(livecaption_models PUBLIC Qt6::Core)
+target_link_libraries(livecaption_models PUBLIC Qt6::Core Qt6::Network)
 qt_add_resources(livecaption_models "registry"
     PREFIX "/"
     BASE "${CMAKE_CURRENT_SOURCE_DIR}"
@@ -127,9 +128,11 @@ qt_add_resources(livecaption_models "registry"
 add_library(livecaption_ui STATIC
     src/ui/captionwindow.cpp
     src/ui/livetranscriber.cpp
+    src/ui/appsettings.cpp
+    src/ui/settingsdialog.cpp
 )
 target_include_directories(livecaption_ui PUBLIC src/ui)
-target_link_libraries(livecaption_ui PUBLIC livecaption_core Qt6::Widgets)
+target_link_libraries(livecaption_ui PUBLIC livecaption_core livecaption_models Qt6::Widgets)
 
 # ---------------------------------------------------------------------------
 # 可执行程序