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.

documentation/BIG_PICTURE_MODE.md

@@ -2,6 +2,36 @@
 
 Nebula Browser includes a **Big Picture Mode** - a controller-friendly, console-style interface designed for Steam Deck, handheld devices, and living room setups.
 
+## ⚠️ Steam Deck: Disabling Mouse Emulation
+
+If Steam is emulating mouse/keyboard input with the joysticks (overriding native controller support), you need to configure Steam Input:
+
+### Quick Fix - Steam Launch Options
+
+Add this to your Steam launch options (Right-click → Properties → Launch Options):
+
+```bash
+SDL_GAMECONTROLLER_ALLOW_STEAM_VIRTUAL_GAMEPAD=0 STEAM_INPUT_ENABLE_VIRTUAL_GAMEPAD=0 %command% --big-picture
+```
+
+### Recommended Fix - Disable Steam Input Per-Game
+
+1. In Steam, right-click **Nebula** → **Properties** → **Controller**
+2. Set **"Override for Nebula"** to **"Disable Steam Input"**
+3. This completely disables Steam's input emulation for this app
+
+### Using the Steam Deck Launcher Script
+
+For the easiest setup, use the included launcher script:
+
+1. Set Steam launch options to: `./start-steamdeck.sh`
+
+The script automatically sets all necessary environment variables.
+
+See [README-STEAM.md](../README-STEAM.md) for detailed troubleshooting.
+
+---
+
 ## Features
 
 ### 🎮 Controller Support