feat: Material Icons, customizable app name & dark/light mode toggle

- Add Google Material Icons integration with smart_toy robot icon
- Implement app_settings database table for persistent configuration
- Add App Name customization in Settings (changes navigation & title)
- Add Dark/Light Mode theme switcher
  - Dark Mode: Lightened black (#0f0f0f) with blue accents (#0a84ff)
  - Light Mode: Clean white/gray with red accents (#ef4444)
- Create context_processor for global app_settings injection
- Redesign Settings page with new App Design section
- Optimize CSS: 724 additions, reduced complexity
- Remove outdated agent reminders (70 lines cleanup)

templates/settings.html

@@ -4,11 +4,47 @@
 {% block content %}
 <div class="page-header mb-4">
   <h1 class="page-title">Einstellungen</h1>
-  <p class="page-subtitle text-muted">Poller-Konfiguration &amp; System-Status</p>
+  <p class="page-subtitle text-muted">App-Design, Poller-Konfiguration &amp; System-Status</p>
 </div>
 
 <div class="row g-4">
 
+  <!-- App-Einstellungen (Name & Theme) -->
+  <div class="col-lg-6">
+    <div class="card h-100">
+      <div class="card-header">
+        <span class="material-icons me-2" style="font-size:18px;vertical-align:middle">palette</span> App-Design
+      </div>
+      <div class="card-body">
+        <form method="POST">
+          <div class="mb-4">
+            <label class="form-label fw-semibold">App-Name</label>
+            <input type="text" class="form-control" name="app_name"
+                   value="{{ app_name }}" placeholder="Frankenbot" required>
+            <div class="form-text">Der Name erscheint in der Navigation und im Browser-Tab.</div>
+          </div>
+
+          <div class="mb-4">
+            <label class="form-label fw-semibold">Design-Theme</label>
+            <select class="form-select" name="theme" required>
+              <option value="dark" {% if theme == 'dark' %}selected{% endif %}>Dark Mode (Blau)</option>
+              <option value="light" {% if theme == 'light' %}selected{% endif %}>Light Mode (Rot)</option>
+            </select>
+            <div class="form-text">
+              <strong>Dark Mode:</strong> Dunkler Hintergrund mit blauen Akzenten<br>
+              <strong>Light Mode:</strong> Heller Hintergrund mit roten Akzenten
+            </div>
+          </div>
+
+          <div class="d-flex gap-2">
+            <button type="submit" class="btn btn-primary">Design speichern</button>
+            <a href="/settings" class="btn btn-outline-secondary">Zurücksetzen</a>
+          </div>
+        </form>
+      </div>
+    </div>
+  </div>
+
   <!-- Poller-Einstellungen -->
   <div class="col-lg-6">
     <div class="card h-100">