feat: add home page with search/unassigned tables; center login card

- Add home view as default: search by inv number, unassigned equipment
  table with auditory assignment, search results (assigned) table
- Add inv_number query param to GET /oboruds/ for backend search
- Center login card vertically and horizontally via flexbox

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/login.html

@@ -7,7 +7,9 @@
     <link rel="stylesheet" href="/app/bootstrap.min.css" />
     <link rel="stylesheet" href="/app/styles.css" />
     <style>
-      .login-card { max-width: 420px; margin: 40px auto; }
+      body { display: flex; flex-direction: column; min-height: 100vh; }
+      main { flex: 1; display: flex; align-items: center; justify-content: center; width: 100%; }
+      .login-card { max-width: 420px; width: 100%; margin: 0; }
       .muted { color: #6c757d; font-size: 0.9rem; }
     </style>
   </head>
@@ -17,7 +19,7 @@
       <h2>Авторизация</h2>
     </header>
 
-    <main class="container">
+    <main>
       <div class="card login-card">
         <div class="card-body">
           <h5 class="card-title mb-3">Вход</h5>