Phase 3a Frontend-Hygiene: Toast statt alert/confirm

- src/static/css/style.css: Toast-Styles (.toast-container, .toast,
  Varianten info/success/warning/error, Animations)
- src/static/dashboard.html: <div id=toastContainer> vor </body>,
  Cancel-Button im Confirm-Modal bekommt id=confirmCancelBtn
- src/static/js/app.js:
  - showToast(msg, type) neu - links oben, autoclose 3.5s (error: 6s)
  - showConfirm(title, text, callback?) jetzt Promise<boolean>-fähig
    (Backwards-compat: Legacy-Callback wird bei OK weiter aufgerufen)
  - Cancel/Close-Hooks am modalConfirm setzen Promise auf false
- alle 18 alert() in app.js / source-health.js / sources.js durch
  showToast(msg, type) ersetzt (type je nach Kontext error/success/warning/info)
- 2 confirm() in source-health.js durch await showConfirm() ersetzt

src/static/dashboard.html

@@ -672,7 +672,7 @@
         <p class="confirm-text" id="confirmText"></p>
       </div>
       <div class="modal-footer">
-        <button class="btn btn-secondary" onclick="closeModal('modalConfirm')">Abbrechen</button>
+        <button class="btn btn-secondary" id="confirmCancelBtn" onclick="closeModal('modalConfirm')">Abbrechen</button>
         <button class="btn btn-danger" id="confirmOkBtn">Bestätigen</button>
       </div>
     </div>
@@ -682,5 +682,6 @@
   <script src="/static/js/sources.js"></script>
   <script src="/static/js/source-health.js"></script>
   <script src="/static/js/audit.js"></script>
+  <div id="toastContainer" class="toast-container" aria-live="polite" aria-atomic="true"></div>
 </body>
 </html>