feat: compact dashboard + login system for admin pages

Dashboard:
- Much smaller cards (col-xl-2), compact badges
- Short labels (Buero, HO, DND, OoO, etc.)
- Optimized for small screens

Login:
- Password-protected admin pages (all except dashboard)
- Dashboard remains public (read-only status view)
- Login page with redirect back to requested page
- Session cookie (24h, regenerated on restart)
- Password set via Settings page (SHA256 hashed)
- Sidebar shows only Dashboard when not logged in
- Login/Logout link in sidebar footer

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

templates/settings.html

@@ -77,6 +77,24 @@
                 </div>
             </div>
 
+            <div class="card mb-4">
+                <div class="card-header">
+                    <h6 class="mb-0"><i class="bi bi-shield-lock"></i> Zugangsschutz</h6>
+                </div>
+                <div class="card-body">
+                    <div class="mb-3">
+                        <label class="form-label">Admin-Passwort</label>
+                        <input type="password" name="admin_password" class="form-control"
+                               placeholder="Neues Passwort eingeben (leer = nicht aendern)">
+                        {% if settings.admin_password %}
+                        <small class="text-success"><i class="bi bi-check-circle"></i> Passwort ist gesetzt</small>
+                        {% else %}
+                        <small class="text-warning"><i class="bi bi-exclamation-triangle"></i> Kein Passwort gesetzt - alle Seiten sind frei zugaenglich</small>
+                        {% endif %}
+                    </div>
+                </div>
+            </div>
+
             <div class="card">
                 <div class="card-header">
                     <h6 class="mb-0"><i class="bi bi-file-earmark-spreadsheet"></i> AIDA Zeiterfassung</h6>