feat(task): undo toast for completing — mis-taps are one tap to take back

completeTask returns an undo closure that restores status + sort slot and
trashes the occurrence a repeat spawned. One global snackbar (5s,
replace-on-new) mounted in the root layout; every checkbox and Clarify's
save-as-done go through completeWithUndo. bun tests for the store.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YE5FmDZmFnWxH6RXCZLcjj

app/src/lib/store.ts

@@ -136,6 +136,11 @@ export function createTask(data: Partial<Task> & { title: string }): Task {
 }
 
 export function updateTask(id: string, patch: Partial<Task>) {
+  applyTaskUpdate(id, patch);
+}
+
+/** Applies a patch; returns the next occurrence if completing spawned one. */
+function applyTaskUpdate(id: string, patch: Partial<Task>): Task | null {
   const before = state.tasks.find((t) => t.id === id);
   setState({
     tasks: state.tasks.map((t) => {
@@ -152,11 +157,28 @@ export function updateTask(id: string, patch: Partial<Task>) {
   // done row later leaves the spawn in place — it's one tap to trash, versus
   // silently deleting a row the user may have edited.
   if (before && before.repeat && before.status !== "done" && patch.status === "done") {
-    spawnNextOccurrence(before);
+    return spawnNextOccurrence(before);
   }
+  return null;
 }
 
-function spawnNextOccurrence(t: Task) {
+/**
+ * Complete a task and hand back an undo. Undo puts the task back where it was
+ * (same status and sort slot) and trashes the occurrence a repeat spawned, so
+ * a mis-tap leaves no trace. Returns null when there's nothing to complete.
+ */
+export function completeTask(id: string, revertTo?: Task["status"]): (() => void) | null {
+  const before = state.tasks.find((t) => t.id === id);
+  if (!before || before.status === "done") return null;
+  const spawned = applyTaskUpdate(id, { status: "done" });
+  const back = revertTo ?? before.status;
+  return () => {
+    updateTask(id, { status: back });
+    if (spawned) trashTask(spawned.id);
+  };
+}
+
+function spawnNextOccurrence(t: Task): Task {
   const today = todayIso();
   const due = t.due_date ? nextOccurrence(t.due_date, t.repeat!, today) : null;
   // Keep the same lead time between defer and due (e.g. "surfaces 3 days before").
@@ -168,7 +190,7 @@ function spawnNextOccurrence(t: Task) {
         : t.defer_date
           ? nextOccurrence(t.defer_date, t.repeat!, today)
           : null;
-  createTask({
+  return createTask({
     title: t.title,
     notes: t.notes,
     status: t.status,