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/css/style.css

@@ -790,3 +790,44 @@ tr:hover td {
 .audit-diff .diff-new { color: #2ecc71; word-break: break-word; }
 .token-budget-bar.over-limit { background: repeating-linear-gradient(45deg, #c0392b, #c0392b 6px, #962d22 6px, #962d22 12px); }
 input[type="date"].filter-select { padding: 6px 10px; }
+
+/* === Toast-Notifications (Phase 3) === */
+.toast-container {
+    position: fixed;
+    top: 24px;
+    right: 24px;
+    z-index: 9999;
+    display: flex;
+    flex-direction: column;
+    gap: 10px;
+    max-width: 380px;
+    pointer-events: none;
+}
+.toast {
+    background: #1e293b;
+    border: 1px solid #334155;
+    border-left-width: 4px;
+    border-radius: 8px;
+    padding: 12px 16px;
+    color: #e2e8f0;
+    font-size: 14px;
+    line-height: 1.4;
+    box-shadow: 0 8px 24px rgba(0, 0, 0, 0.3);
+    pointer-events: auto;
+    animation: toast-in 0.18s ease-out;
+}
+.toast.toast-out {
+    animation: toast-out 0.18s ease-in forwards;
+}
+.toast-info    { border-left-color: #3b82f6; }
+.toast-success { border-left-color: #10b981; }
+.toast-warning { border-left-color: #f59e0b; }
+.toast-error   { border-left-color: #ef4444; }
+@keyframes toast-in {
+    from { opacity: 0; transform: translateX(20px); }
+    to   { opacity: 1; transform: translateX(0); }
+}
+@keyframes toast-out {
+    from { opacity: 1; transform: translateX(0); }
+    to   { opacity: 0; transform: translateX(20px); }
+}