feat: add light/dark theme, RU/EN i18n, instructions page, green/red monitoring

- Light and dark theme toggle with localStorage persistence
- Russian and English language support (full i18n)
- Instructions & cheat sheet modal with install/SSH/API/commands
- Green pulsing dot for online, red for offline servers
- Color-coded metrics: green (ok), yellow (warn >70%), red (crit >90%)
- Top bar on server cards: green=online, red=offline

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

server/static/js/theme.js

@@ -0,0 +1,16 @@
+let currentTheme = localStorage.getItem('vps_theme') || 'dark';
+
+function applyTheme(theme) {
+    currentTheme = theme;
+    localStorage.setItem('vps_theme', theme);
+    document.documentElement.setAttribute('data-theme', theme);
+    const btn = document.getElementById('themeToggleBtn');
+    if (btn) btn.textContent = theme === 'dark' ? '☀️' : '🌙';
+}
+
+function toggleTheme() {
+    applyTheme(currentTheme === 'dark' ? 'light' : 'dark');
+}
+
+// Apply on load
+document.addEventListener('DOMContentLoaded', () => applyTheme(currentTheme));