feat: implementar 12 mejoras, tests, docs y optimizaciones

- Fase A: license templates, search history, cost estimator
- Fase B: import URL, bulk ZIP, batch download
- Fase C: comparison mode, mesh validation, measurement tool
- Fase D: cross-section clipping, overhang heatmap, layer animation
- Refactor Pydantic/SQLAlchemy warnings
- 24 tests pytest
- README actualizado
- WebP thumbnails, lazy loading, cache headers

static/js/theme.js

@@ -0,0 +1,39 @@
+(function() {
+  const STORAGE_KEY = 'stl-repo-theme';
+
+  function getTheme() {
+    return localStorage.getItem(STORAGE_KEY) || 'dark';
+  }
+
+  function setTheme(theme) {
+    localStorage.setItem(STORAGE_KEY, theme);
+    applyTheme(theme);
+  }
+
+  function applyTheme(theme) {
+    const html = document.documentElement;
+    if (theme === 'light') {
+      html.classList.add('light-mode');
+    } else {
+      html.classList.remove('light-mode');
+    }
+    window.dispatchEvent(new CustomEvent('themechange', { detail: theme }));
+  }
+
+  function toggleTheme() {
+    const current = getTheme();
+    setTheme(current === 'dark' ? 'light' : 'dark');
+  }
+
+  // Expose globally
+  window.getTheme = getTheme;
+  window.setTheme = setTheme;
+  window.toggleTheme = toggleTheme;
+
+  // Apply on load
+  document.addEventListener('DOMContentLoaded', () => {
+    applyTheme(getTheme());
+    const btn = document.getElementById('theme-toggle');
+    if (btn) btn.addEventListener('click', toggleTheme);
+  });
+})();