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/snippets.html

@@ -110,7 +110,7 @@
                 <h3 class="font-bold text-lg text-on-surface">Store Custom Snippet</h3>
                 <button @click="open = false" class="text-outline hover:text-primary p-1 rounded">Cancel</button>
               </div>
-              <form hx-post="/snippets" hx-target="#snippet-grid" hx-swap="outerHTML"
+              <form hx-post="/snippets" hx-target="#snippet-grid" hx-swap="innerHTML"
                     @submit="open = false" class="space-y-4 text-sm">
                 <div class="flex flex-col gap-1">
                   <label class="text-xs font-bold text-on-surface-variant">SNIPPET TITLE</label>