Add Docker/Compose setup and fix subpath routing for /toroid deployment

- Add Dockerfile, .dockerignore, and docker-compose.yml to containerize the app
- Add toroid.service systemd unit (uses docker compose)
- Mount presets/ as a bind volume for persistence outside the container
- Fix all fetch() calls in templates to use relative paths (no leading /)
  so they resolve correctly when served under /toroid/ via nginx proxy

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

templates/manual.html

@@ -266,7 +266,7 @@
     <div class="container">
         <div class="header">
             <h1>🔧 Manual Simulation</h1>
-            <a href="/" class="nav-link">⚡ Go to Optimizer</a>
+            <a href="./" class="nav-link">⚡ Go to Optimizer</a>
         </div>
         <p class="subtitle">Directly specify tap settings and input voltage to simulate transformer performance</p>
 
@@ -455,7 +455,7 @@
             loading.classList.remove('hidden');
 
             try {
-                const response = await fetch('/api/simulate', {
+                const response = await fetch('api/simulate', {
                     method: 'POST',
                     headers: {
                         'Content-Type': 'application/json',
@@ -523,7 +523,7 @@
         // Load transformer info on page load
         async function loadTransformerInfo() {
             try {
-                const response = await fetch('/api/transformer_info');
+                const response = await fetch('api/transformer_info');
                 const data = await response.json();
 
                 // Update tap options based on actual transformer