feat: Redesign Dark Mode with balanced purple theme

- Change Dark Mode accent from blue to purple (#a855f7)
- Balance dark backgrounds to match Light Mode structure
  - bg-base: #1a1a1a (was #0f0f0f - too dark)
  - bg-surface: #212121 (more balanced)
  - Improved contrast and readability
- Update borders to be more visible (12% vs 8% opacity)
- Adjust agent colors for purple theme:
  - Orchestrator: Purple
  - Researcher: Green
  - Negotiator: Orange
  - AR Manager: Yellow
- Update Settings description: 'Violett' instead of 'Blau'
- Dark Mode now mirrors Light Mode balance (just inverted)

templates/settings.html

@@ -27,11 +27,11 @@
           <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="dark" {% if theme == 'dark' %}selected{% endif %}>Dark Mode (Violett)</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>Dark Mode:</strong> Ausgewogene dunkle Grautöne mit violetten Akzenten<br>
               <strong>Light Mode:</strong> Heller Hintergrund mit roten Akzenten
             </div>
           </div>