feat(app): per-variant single-instance lock so full node + Lite can run together

The single-instance lock hardcoded the name "obsidiandragon" for both variants,
so launching ObsidianDragonLite while ObsidianDragon was running (or vice-versa)
was refused with "Another instance is already running". Nothing else actually
required them to be exclusive — DRAGONX_APP_NAME already gives each variant its
own config dir (settings / wallets index / address book / chat db), the full
node's daemon lives under ~/.hush/DRAGONX with no Lite counterpart, and the lock
guards no cross-instance IPC (the payment URI is handled locally).

Key the lock per variant (obsidiandragon / obsidiandragonlite) — the Windows
named mutex already derives from the same name, so it's fixed on both platforms.
Each variant still enforces a single instance of itself. Also make the
already-running message report the actual variant (DRAGONX_APP_NAME) and use
MessageBoxA so it can. mingw-verified.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/main.cpp

@@ -439,8 +439,13 @@ static bool InitImGui(SDL_Window* window, SDL_GLContext gl_context);
 static void Shutdown(SDL_Window* window, SDL_GLContext gl_context);
 #endif
 
-// Global single instance lock
+// Global single instance lock. Keyed PER VARIANT so the full node and Lite can run side by side —
+// their config dirs are already separate (DRAGONX_APP_NAME), so only this lock kept them apart.
+#ifdef DRAGONX_LITE_BUILD
+static dragonx::util::SingleInstance g_single_instance("obsidiandragonlite");
+#else
 static dragonx::util::SingleInstance g_single_instance("obsidiandragon");
+#endif
 
 // Check for payment URI in command line args
 static std::string findPaymentURI(int argc, char* argv[])
@@ -764,11 +769,12 @@ int main(int argc, char* argv[])
 
     // Check for existing instance
     if (!g_single_instance.tryLock()) {
-        fprintf(stderr, "Another instance of ObsidianDragon is already running.\n");
+        fprintf(stderr, "Another instance of %s is already running.\n", DRAGONX_APP_NAME);
         DEBUG_LOGF("Please close the existing instance first.\n");
 #ifdef _WIN32
-        MessageBoxW(nullptr, L"Another instance of ObsidianDragon is already running.\nPlease close it first.",
+        const std::string msg = std::string("Another instance of ") + DRAGONX_APP_NAME +
-                    L"ObsidianDragon", MB_OK | MB_ICONINFORMATION);
+                                " is already running.\nPlease close it first.";
+        MessageBoxA(nullptr, msg.c_str(), DRAGONX_APP_NAME, MB_OK | MB_ICONINFORMATION);
 #endif
         return 1;
     }