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/index.html

@@ -17,12 +17,15 @@
   <div class="flex min-h-screen">
     {{template "nav" .}}
     <div class="flex-1 flex flex-col overflow-hidden">
-      <header class="h-14 border-b border-outline-variant/30 flex items-center gap-3 px-6 bg-white/70 backdrop-blur-md">
-        <h1 class="text-xl font-bold tracking-tight text-on-surface">Hosts</h1>
-        <span class="text-xs text-on-surface-variant mt-0.5">Manage connections, monitor uptime, and deploy</span>
+      <header class="h-14 border-b border-outline-variant/30 flex items-center px-6 bg-white/70 backdrop-blur-md">
+        <h1 class="text-lg font-semibold text-on-surface">Hosts</h1>
       </header>
       <main id="main-content" class="flex-1 overflow-auto p-6">
         <div id="dashboard-page" class="max-w-7xl mx-auto space-y-8">
+          <div class="border-b border-outline-variant/30 pb-4">
+            <h2 class="text-xl font-bold text-on-surface tracking-tight">Hosts</h2>
+            <p class="text-xs text-on-surface-variant mt-0.5">Manage connections, monitor uptime, and deploy</p>
+          </div>
           <div class="grid grid-cols-1 md:grid-cols-12 gap-6">
             <div class="md:col-span-8 p-6 rounded-2xl bg-white border border-outline-variant shadow-sm flex items-center justify-between overflow-hidden relative group">
               <div class="relative z-10 space-y-2">