fix(cache): bust browser cache for JS and i18n files after updates

After a container update, browsers serve stale app.js and lang/*.json
from cache, causing old UI code and missing translations to appear.

- serve_index() now reads the git commit hash and injects ?v=COMMIT into
  all static asset URLs (app.js, i18n.js, styles.css) in index.html
- window.STATIC_VERSION is injected into the page so i18n.js can append
  the same version to lang/*.json fetch calls
- index.html itself is served with Cache-Control: no-cache so the browser
  always revalidates it and picks up new asset URLs on next load

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

static/js/i18n.js

@@ -27,7 +27,8 @@ function detectLanguage() {
 async function loadLanguage(lang) {
     if (translations[lang]) return;
     try {
-        const resp = await fetch(`/static/lang/${lang}.json`);
+        const v = window.STATIC_VERSION ? `?v=${window.STATIC_VERSION}` : '';
+        const resp = await fetch(`/static/lang/${lang}.json${v}`);
         if (!resp.ok) throw new Error(`HTTP ${resp.status}`);
         translations[lang] = await resp.json();
     } catch (err) {