perf(startup): drop the unused SDL gamepad subsystem and nav flag

Startup initialised SDL's gamepad subsystem and turned on ImGui gamepad
navigation, but the wallet is never driven by a gamepad. Initialising
the subsystem enumerates attached controllers and loads the mapping
database for nothing.

- SDL now starts with the video subsystem only.
- ImGui keeps keyboard navigation; the gamepad navigation flag is
  removed from both the DX11 and the OpenGL setup paths.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main.cpp

@@ -2432,7 +2432,7 @@ static bool InitSDL()
     // sub-15 ms waits (vsync pacing, the 200 ms idle wait, >=50 ms thread sleeps), so let the system
     // timer relax. Ignored on non-Windows platforms; a false return (env override) is harmless.
     SDL_SetHint(SDL_HINT_TIMER_RESOLUTION, "0");
-    if (!SDL_Init(SDL_INIT_VIDEO | SDL_INIT_GAMEPAD)) {
+    if (!SDL_Init(SDL_INIT_VIDEO)) {
         fprintf(stderr, "Error: SDL_Init(): %s\n", SDL_GetError());
         return false;
     }
@@ -2457,8 +2457,6 @@ static bool InitImGui(SDL_Window* window, dragonx::platform::DX11Context& dx)
     
     // Enable keyboard navigation
     io.ConfigFlags |= ImGuiConfigFlags_NavEnableKeyboard;
-    // Enable gamepad navigation
-    io.ConfigFlags |= ImGuiConfigFlags_NavEnableGamepad;
     // Enable multi-viewport (windows can float outside main window)
     io.ConfigFlags |= ImGuiConfigFlags_ViewportsEnable;
 
@@ -2519,8 +2517,6 @@ static bool InitImGui(SDL_Window* window, SDL_GLContext gl_context)
     
     // Enable keyboard navigation
     io.ConfigFlags |= ImGuiConfigFlags_NavEnableKeyboard;
-    // Enable gamepad navigation
-    io.ConfigFlags |= ImGuiConfigFlags_NavEnableGamepad;
     // Enable multi-viewport (windows can float outside main window)
     io.ConfigFlags |= ImGuiConfigFlags_ViewportsEnable;