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/sftp_pane.html

@@ -38,13 +38,13 @@
           <div class="flex items-center justify-end gap-1.5 opacity-0 group-hover:opacity-100 transition-opacity">
             {{if ne .Type "folder"}}
             {{if eq $.Pane "local"}}
-            <button @click="document.querySelector('#sftp-page').__x.$data.toast = 'Uploading {{.Name}}...'; document.querySelector('#sftp-page').__x.$data.showToast = true"
+            <button @click="$dispatch('toast-message', { message: 'Uploading {{.Name}}...' })"
                     class="p-1 hover:bg-primary/10 rounded text-primary transition-colors" title="Upload to remote">
               <svg class="w-3.5 h-3.5" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M4 16v2a2 2 0 002 2h12a2 2 0 002-2v-2M7 10l5-5m0 0l5 5m-5-5v12"/></svg>
             </button>
             {{end}}
             {{if eq $.Pane "remote"}}
-            <button @click="document.querySelector('#sftp-page').__x.$data.toast = 'Downloading {{.Name}}...'; document.querySelector('#sftp-page').__x.$data.showToast = true"
+            <button @click="$dispatch('toast-message', { message: 'Downloading {{.Name}}...' })"
                     class="p-1 hover:bg-primary/10 rounded text-primary transition-colors" title="Download to local">
               <svg class="w-3.5 h-3.5" fill="none" stroke="currentColor" viewBox="0 0 24 24"><path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M12 10v6m0 0l-3-3m3 3l3-3m2 8H7a2 2 0 01-2-2V5a2 2 0 012-2h5.586a1 1 0 01.707.293l5.414 5.414a1 1 0 01.293.707V19a2 2 0 01-2 2z"/></svg>
             </button>