feat: compact UI with theme switcher (light/dark/mono)

- Redesign download cards to be significantly more compact
- Add CSS custom properties for theming
- Add theme switcher (Light, Dark, Mono) with localStorage persistence
- Update README with environment variable Docker deployment docs
- Update Docker Compose example to use environment: instead of volume mount

public/index.html

@@ -32,6 +32,11 @@
             <header class="app-header">
                 <h1>sofarr</h1>
                 <div class="header-controls">
+                    <div class="theme-switcher">
+                        <button class="theme-btn active" data-theme="light">Light</button>
+                        <button class="theme-btn" data-theme="dark">Dark</button>
+                        <button class="theme-btn" data-theme="mono">Mono</button>
+                    </div>
                     <div class="refresh-control">
                         <label for="refresh-rate">Refresh:</label>
                         <select id="refresh-rate">