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/tsconfig.json

@@ -16,5 +16,8 @@
     "**/*.tsx",
     ".expo/types/**/*.ts",
     "expo-env.d.ts"
+  ],
+  "exclude": [
+    "**/*.test.ts"
   ]
 }