fix: add missing _content partial templates for HTMX nav (terminal, sftp, settings, brief)

- Create terminal_content.html, sftp_content.html, settings_content.html, brief_content.html
- Update full page templates to use partial templates via {{template}}
- Handlers already rendered these templates on HX-Request, but files were missing → 500
- All pages now return 200 for both full page and HTMX nav requests

app/backend/static/js/clipboard.js

@@ -1,10 +1,14 @@
-function copyToClipboard(text) {
+document.addEventListener('click', function(e) {
+  const btn = e.target.closest('[data-copy]');
+  if (!btn) return;
+  const text = btn.getAttribute('data-copy');
+  if (!text) return;
   if (navigator.clipboard && navigator.clipboard.writeText) {
     navigator.clipboard.writeText(text).catch(() => fallbackCopy(text));
   } else {
     fallbackCopy(text);
   }
-}
+});
 
 function fallbackCopy(text) {
   const ta = document.createElement('textarea');