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.

main.js

@@ -1,3 +1,38 @@
+// =============================================================================
+// STEAM DECK / STEAMOS CONTROLLER INPUT FIX
+// =============================================================================
+// These environment variables MUST be set before Electron/Chromium initializes.
+// They signal to Steam's input layer that this application handles its own
+// controller input and should NOT have mouse/keyboard emulation applied.
+//
+// Without these, Steam assumes the app needs Desktop mouse emulation when running
+// in Game Mode, which overrides the app's native gamepad support.
+// =============================================================================
+
+// Tell SDL (and by extension Steam Input) that this app uses the gamepad API
+// SDL_GAMECONTROLLERCONFIG is used by SDL to know about controllers
+process.env.SDL_GAMECONTROLLERCONFIG = process.env.SDL_GAMECONTROLLERCONFIG || '';
+
+// Signal that this app handles gamepad input natively
+// This prevents Steam from applying mouse emulation in Game Mode
+process.env.SDL_GAMECONTROLLER_ALLOW_STEAM_VIRTUAL_GAMEPAD = '1';
+
+// Prevent Steam from remapping the controller to keyboard/mouse
+// Setting to '1' tells Steam we want raw controller access
+process.env.SDL_GAMECONTROLLER_IGNORE_DEVICES = '';
+
+// Disable Steam's overlay input hooks for this process if possible
+process.env.SteamNoOverlayUIDrawing = process.env.SteamNoOverlayUIDrawing || '0';
+
+// Tell Steam Input we're a native controller app
+// When STEAM_INPUT_ENABLE_VIRTUAL_GAMEPAD is 0, Steam won't virtualize the gamepad
+if (!process.env.STEAM_INPUT_ENABLE_VIRTUAL_GAMEPAD) {
+  process.env.STEAM_INPUT_ENABLE_VIRTUAL_GAMEPAD = '0';
+}
+
+// Hint that this is a game/controller-focused app
+process.env.SDL_HINT_JOYSTICK_ALLOW_BACKGROUND_EVENTS = '1';
+
 const { app, BrowserWindow, ipcMain, session, screen, shell, dialog, Menu, clipboard, webContents } = require('electron');
 const { autoUpdater } = require('electron-updater');
 const { pathToFileURL } = require('url');
@@ -61,6 +96,30 @@ try {
   // Non-fatal: some environments may not allow commandLine changes at this time.
 }
 
+// =============================================================================
+// GAMEPAD / CONTROLLER CHROMIUM FLAGS
+// =============================================================================
+// Enable native gamepad support in Chromium - helps with Steam Deck compatibility
+try {
+  // Enable raw gamepad access (bypasses Steam's virtualization when possible)
+  app.commandLine.appendSwitch('enable-gamepad-extensions');
+  
+  // Ensure the Gamepad API is enabled and working
+  app.commandLine.appendSwitch('enable-blink-features', 'GamepadExtensions');
+  
+  // On Linux/Steam Deck, this can help with gamepad detection
+  if (process.platform === 'linux') {
+    // Disable Chromium's sandbox for gamepad access if having issues
+    // (Only needed in some SteamOS configurations)
+    // app.commandLine.appendSwitch('no-sandbox');
+    
+    // Use the system's gamepad config rather than Chromium's built-in
+    app.commandLine.appendSwitch('enable-features', 'WebGamepad');
+  }
+} catch (e) {
+  console.warn('[Gamepad] Failed to set Chromium gamepad flags:', e.message);
+}
+
 // Configure GPU settings before app is ready
 gpuConfig.configure();