Improve Steam Deck controller support and documentation

Adds environment variable setup and early gamepad initialization to main.js and preload.js to ensure native controller support on Steam Deck and SteamOS. Updates README-STEAM.md and BIG_PICTURE_MODE.md with detailed instructions for disabling Steam Input emulation and troubleshooting. Introduces start-steamdeck.sh launcher script to automate environment configuration for Steam Deck users.

preload.js

@@ -224,6 +224,29 @@ window.addEventListener('beforeunload', () => {
   }
 });
 
+// =============================================================================
+// EARLY GAMEPAD INITIALIZATION - Critical for Steam Deck
+// =============================================================================
+// Initialize gamepad polling as EARLY as possible to signal Steam Input
+// that this app handles controller input natively. This MUST happen before
+// Steam decides to apply mouse/keyboard emulation.
+// 
+// We try to initialize immediately when preload runs, not waiting for DOMContentLoaded,
+// because Steam's input layer makes decisions very early in the process lifecycle.
+// =============================================================================
+
+// Try immediate initialization (works in most Electron contexts)
+try {
+  if (typeof navigator !== 'undefined' && navigator.getGamepads) {
+    // Start polling immediately - this is the key signal to Steam
+    initGamepadHandler();
+    console.log('[NebulaGamepad] Early initialization successful - Steam should recognize controller input');
+  }
+} catch (e) {
+  // Will retry on DOMContentLoaded
+  console.log('[NebulaGamepad] Early init deferred, will retry on DOM ready');
+}
+
 // =============================================================================
 // DOM READY & INITIALIZATION
 // =============================================================================
@@ -234,8 +257,10 @@ window.addEventListener('DOMContentLoaded', () => {
   domReady = true;
   console.log("Browser UI loaded.");
   
-  // Initialize gamepad handler for Steam Deck/SteamOS support
-  initGamepadHandler();
+  // Re-initialize gamepad handler if early init failed
+  if (!gamepadState.initialized) {
+    initGamepadHandler();
+  }
 });
 
 // Optimized API exposure with error handling and caching