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/views/nav.html

@@ -1,6 +1,6 @@
 {{define "nav"}}
 <nav class="w-[260px] min-h-screen bg-white/70 backdrop-blur-md border-r border-outline-variant/30 flex flex-col shrink-0">
-  <div class="p-4 border-b border-outline-variant/20">
+  <div class="p-3 border-b border-outline-variant/20">
     <div class="flex items-center gap-2">
       <div class="w-8 h-8 rounded-lg bg-primary flex items-center justify-center text-white text-sm font-bold">K</div>
       <span class="font-bold text-lg text-on-surface">Hostkeeper</span>