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/keychain.html

@@ -44,7 +44,7 @@
           </div>
 
           <!-- Grid List -->
-          <div id="key-grid" class="grid grid-cols-1 md:grid-cols-2 gap-6">
+          <div class="grid grid-cols-1 md:grid-cols-2 gap-6" id="key-grid">
             {{template "credential_grid" .}}
           </div>
 
@@ -70,7 +70,7 @@
                 <h3 class="font-bold text-lg text-on-surface">Register Secure Credential</h3>
                 <button @click="open = false" class="text-outline hover:text-primary p-1 rounded">Cancel</button>
               </div>
-              <form hx-post="/keys" hx-target="#key-grid" hx-swap="outerHTML"
+              <form hx-post="/keys" hx-target="#key-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">CREDENTIAL NAME / LABEL</label>