feat: reuse tabs across multi-run — no more tab bloat

- tabRegistry preserved across resetState (like seenCodes/accounts)
- New reuseOrCreateTab() helper: navigates existing tab or creates new one
- All steps use reuseOrCreateTab: VPS panel, auth page, mail
- Mail tabs: only activate if alive, don't re-navigate (preserves session)
- Auth tab: reused between step 2→6→8, navigated to new URLs
- VPS tab: reused across runs, re-injected on navigation

sidepanel/sidepanel.css

@@ -541,6 +541,94 @@ header {
 
 .log-line { animation: fadeIn 120ms ease-out; }
 
+/* ============================================================
+   Toast Notifications
+   ============================================================ */
+
+#toast-container {
+  position: fixed;
+  top: 12px;
+  left: 12px;
+  right: 12px;
+  z-index: 1000;
+  display: flex;
+  flex-direction: column;
+  gap: 6px;
+  pointer-events: none;
+}
+
+.toast {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  padding: 10px 12px;
+  border-radius: var(--radius-md);
+  font-size: 12px;
+  font-weight: 500;
+  box-shadow: var(--shadow-md), 0 4px 12px rgba(0,0,0,0.1);
+  pointer-events: auto;
+  animation: toastIn 250ms ease-out;
+  border: 1px solid;
+}
+
+.toast.toast-exit {
+  animation: toastOut 200ms ease-in forwards;
+}
+
+.toast-error {
+  background: var(--red-soft);
+  border-color: var(--red);
+  color: var(--red);
+}
+
+.toast-warn {
+  background: var(--orange-soft);
+  border-color: var(--orange);
+  color: var(--orange);
+}
+
+.toast-success {
+  background: var(--green-soft);
+  border-color: var(--green);
+  color: var(--green);
+}
+
+.toast-info {
+  background: var(--blue-soft);
+  border-color: var(--blue);
+  color: var(--blue);
+}
+
+.toast svg {
+  flex-shrink: 0;
+}
+
+.toast-msg {
+  flex: 1;
+}
+
+.toast-close {
+  background: none;
+  border: none;
+  color: inherit;
+  opacity: 0.6;
+  cursor: pointer;
+  padding: 2px;
+  font-size: 16px;
+  line-height: 1;
+}
+.toast-close:hover { opacity: 1; }
+
+@keyframes toastIn {
+  from { opacity: 0; transform: translateY(-10px) scale(0.96); }
+  to { opacity: 1; transform: translateY(0) scale(1); }
+}
+
+@keyframes toastOut {
+  from { opacity: 1; transform: translateY(0) scale(1); }
+  to { opacity: 0; transform: translateY(-10px) scale(0.96); }
+}
+
 @media (prefers-reduced-motion: reduce) {
   *, *::before, *::after {
     animation-duration: 0.01ms !important;