fix: Adjust chat height for admin bar (#21)

- Added CSS variable --header-height (109px for admin, 73px for regular users)
- Fixed chat container height calculation to account for admin bar
- Fixed mobile sidebar position

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

templates/chat.html

@@ -7,6 +7,11 @@
 {% block extra_css %}
 <style>
     /* Reset dla pełnoekranowego chatu jak ChatGPT/Claude */
+    :root {
+        /* Wysokość nagłówka: 73px navbar + 36px admin bar (jeśli admin) */
+        --header-height: {% if current_user.is_authenticated and current_user.is_admin %}109px{% else %}73px{% endif %};
+    }
+
     html, body {
         overflow: hidden !important; /* Blokada scrollowania strony */
         height: 100% !important;
@@ -21,8 +26,8 @@
         padding: 0 !important;
         display: flex;
         flex-direction: column;
-        height: calc(100vh - 73px) !important; /* Wysokość minus navbar */
-        max-height: calc(100vh - 73px) !important;
+        height: calc(100vh - var(--header-height)) !important; /* Wysokość minus navbar (+ admin bar) */
+        max-height: calc(100vh - var(--header-height)) !important;
         overflow: hidden !important;
     }
 
@@ -568,7 +573,7 @@
         .chat-sidebar {
             display: none;
             position: fixed;
-            top: 73px;
+            top: var(--header-height);
             left: 0;
             bottom: 0;
             z-index: 100;