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.

start-steamdeck.sh

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+#!/bin/bash
+# =============================================================================
+# NEBULA BROWSER - Steam Deck / SteamOS Launch Script
+# =============================================================================
+# This script is designed for launching Nebula on Steam Deck in Game Mode.
+# It sets necessary environment variables to disable Steam's mouse/keyboard
+# emulation so the app's native controller support works properly.
+#
+# Usage:
+#   1. Add Nebula as a Non-Steam game (or via Steamworks)
+#   2. Set launch options to: ./start-steamdeck.sh
+#   OR
+#   3. Use this script's environment variables in Steam Launch Options:
+#      SDL_GAMECONTROLLER_ALLOW_STEAM_VIRTUAL_GAMEPAD=0 %command% --big-picture
+# =============================================================================
+
+cd "$(dirname "$0")"
+
+# =============================================================================
+# STEAM INPUT CONFIGURATION
+# =============================================================================
+# These variables tell Steam's input layer that this app handles controller
+# input natively and should NOT have mouse/keyboard emulation applied.
+
+# Disable Steam's virtual gamepad layer
+export SDL_GAMECONTROLLER_ALLOW_STEAM_VIRTUAL_GAMEPAD=0
+export STEAM_INPUT_ENABLE_VIRTUAL_GAMEPAD=0
+
+# Allow raw gamepad access
+export SDL_GAMECONTROLLER_IGNORE_DEVICES=""
+
+# Allow background gamepad events (useful when app doesn't have focus)
+export SDL_HINT_JOYSTICK_ALLOW_BACKGROUND_EVENTS=1
+
+# Hint to SDL that we're using gamepads natively
+export SDL_GAMECONTROLLERCONFIG="${SDL_GAMECONTROLLERCONFIG:-}"
+
+# =============================================================================
+# NEBULA CONFIGURATION
+# =============================================================================
+
+# Enable Big Picture Mode for controller-friendly UI
+export NEBULA_BIG_PICTURE=1
+
+# Enable GPU acceleration on Linux
+export NEBULA_GPU_ALLOW_LINUX=1
+
+# =============================================================================
+# LAUNCH
+# =============================================================================
+
+# Check if we're in an AppImage/AppDir or dev environment
+if [ -f "./nebula" ]; then
+    # Packaged AppDir
+    exec ./nebula --big-picture "$@"
+elif [ -f "./Nebula" ]; then
+    # Alternate launcher name
+    exec ./Nebula --big-picture "$@"
+elif command -v npm &> /dev/null && [ -f "package.json" ]; then
+    # Development environment
+    npm start -- --big-picture "$@"
+else
+    echo "Error: Could not find Nebula executable or npm"
+    echo "Make sure you're running this script from the Nebula directory"
+    exit 1
+fi