Add fullscreen toggle and wake lock to prevent screen sleep

## Features
- Fullscreen button (⛶) in header to toggle fullscreen mode
- Wake Lock API integration to keep screen on while using the app
- Sun icon (☀) indicator shows wake lock status (bright = active)

## Wake Lock behavior
- Automatically requests wake lock when page loads
- Re-acquires wake lock when returning to the page
- Visual indicator pulses when active
- Gracefully hidden if Wake Lock API not supported

## Fullscreen
- Works on Android Chrome and desktop browsers
- iOS Safari has limited support (no fullscreen API)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

web/service-worker.js

@@ -1,5 +1,5 @@
 // MacroPad PWA Service Worker
-const CACHE_NAME = 'macropad-v2';
+const CACHE_NAME = 'macropad-v3';
 const ASSETS_TO_CACHE = [
     '/',
     '/static/css/styles.css',