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/components/Toast.tsx

@@ -0,0 +1,98 @@
+// One app-wide snackbar, mounted once in the root layout. `showToast` is a
+// plain function so stores and rows can call it without a context hop. A new
+// toast replaces the current one (last action wins) and the timer restarts.
+import { useEffect, useState } from "react";
+import { Pressable, StyleSheet, Text } from "react-native";
+import Animated, { FadeInDown, FadeOutDown } from "react-native-reanimated";
+import { useSafeAreaInsets } from "react-native-safe-area-context";
+import { haptic } from "../lib/haptics";
+import { C, radius, shadow, space, type } from "../lib/theme";
+
+export interface ToastSpec {
+  message: string;
+  action?: { label: string; onPress: () => void };
+}
+
+const DURATION_MS = 5000;
+// Sits on the home FAB's baseline (index.tsx `fab.bottom`) — above the
+// launcher bar, and the right padding leaves the FAB itself uncovered.
+const FAB_BOTTOM = 104;
+const FAB_CLEARANCE = space.xl + 56 + space.md;
+
+let setCurrent: ((t: ToastSpec | null) => void) | null = null;
+
+export function showToast(spec: ToastSpec) {
+  setCurrent?.(spec);
+}
+
+export function ToastHost() {
+  const [toast, setToast] = useState<ToastSpec | null>(null);
+  const insets = useSafeAreaInsets();
+
+  useEffect(() => {
+    setCurrent = setToast;
+    return () => {
+      if (setCurrent === setToast) setCurrent = null;
+    };
+  }, []);
+
+  useEffect(() => {
+    if (!toast) return;
+    const timer = setTimeout(() => setToast(null), DURATION_MS);
+    return () => clearTimeout(timer);
+  }, [toast]);
+
+  if (!toast) return null;
+  const { action } = toast;
+  return (
+    <Animated.View
+      entering={FadeInDown.duration(180)}
+      exiting={FadeOutDown.duration(150)}
+      style={[s.wrap, { bottom: FAB_BOTTOM + insets.bottom }]}
+      pointerEvents="box-none"
+    >
+      <Pressable style={s.toast} onPress={() => setToast(null)}>
+        <Text style={s.message} numberOfLines={1}>
+          {toast.message}
+        </Text>
+        {action && (
+          <Pressable
+            hitSlop={12}
+            onPress={() => {
+              haptic.tap();
+              setToast(null);
+              action.onPress();
+            }}
+            style={({ pressed }) => [s.action, pressed && { opacity: 0.6 }]}
+          >
+            <Text style={s.actionText}>{action.label}</Text>
+          </Pressable>
+        )}
+      </Pressable>
+    </Animated.View>
+  );
+}
+
+const s = StyleSheet.create({
+  wrap: {
+    position: "absolute",
+    left: space.lg,
+    right: FAB_CLEARANCE,
+    alignItems: "flex-start",
+  },
+  toast: {
+    flexDirection: "row",
+    alignItems: "center",
+    gap: space.lg,
+    maxWidth: 420,
+    paddingVertical: space.md,
+    paddingLeft: space.lg,
+    paddingRight: space.md,
+    borderRadius: radius.pill,
+    backgroundColor: C.text,
+    ...shadow.fab,
+  },
+  message: { ...type.row, color: C.bg, flexShrink: 1 },
+  action: { paddingVertical: space.xs, paddingHorizontal: space.sm },
+  actionText: { ...type.row, color: C.accentSoft, fontWeight: "600" },
+});