feat(web): reusable Modal + ConfirmProvider, replace native confirm()

Modal component: portal, ESC to close, Tab focus-trap, focus-on-open
(prefers [data-modal-autofocus]), focus restore, overlay click, scroll lock.
ConfirmProvider exposes useConfirm(): async confirm({...}) as a drop-in for
window.confirm; Enter confirms, ESC cancels. Task/account deletes now use it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MMHQTtnQtQqL8muAXHr9kd

web/src/app.css

@@ -254,6 +254,77 @@
   cursor: not-allowed;
 }
 
+/* ---------- modal ---------- */
+
+.modal-overlay {
+  position: fixed;
+  inset: 0;
+  z-index: 100;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  padding: 24px;
+  background: rgba(4, 7, 5, 0.72);
+  backdrop-filter: blur(2px);
+  animation: modal-fade 0.12s ease-out;
+}
+
+.modal-dialog {
+  width: 100%;
+  max-width: 440px;
+  background: var(--bg-panel-raised);
+  border: 1px solid var(--border-bright);
+  box-shadow: 0 18px 48px rgba(0, 0, 0, 0.55);
+  padding: 22px 24px;
+  outline: none;
+  animation: modal-rise 0.14s ease-out;
+}
+
+.modal-title {
+  font-family: var(--font-mono);
+  font-size: 12px;
+  letter-spacing: 0.14em;
+  text-transform: uppercase;
+  color: var(--accent);
+  margin-bottom: 14px;
+}
+
+.confirm-message {
+  margin: 0 0 20px;
+  color: var(--fg);
+  font-size: 14px;
+  line-height: 1.5;
+}
+
+.modal-actions {
+  display: flex;
+  justify-content: flex-end;
+  gap: 10px;
+}
+
+.btn-danger {
+  border-color: var(--fail-dim);
+  color: var(--fail);
+}
+
+.btn-danger:hover:not(:disabled) {
+  background: var(--fail-dim);
+  color: var(--fail);
+}
+
+@keyframes modal-fade {
+  from {
+    opacity: 0;
+  }
+}
+
+@keyframes modal-rise {
+  from {
+    opacity: 0;
+    transform: translateY(6px);
+  }
+}
+
 /* ---------- buttons ---------- */
 
 .btn {