test: add e2e tests for todo CRUD, filtering, and persistence

[kashish2508]

Adds Playwright end-to-end tests for the todo CRUD, filtering, and localStorage persistence features introduced in feature/todo-persistence-filter.

1. Todo CRUD — tests/todo-crud.spec.ts

• Gap identified: The new todo section (add, toggle, delete) had zero test coverage.
• User behavior: A user adds a new task via the input form, toggles it complete/incomplete via checkbox, deletes it, and sees the items-left counter update correctly at each step.

2. Todo Filtering & Clear Completed — tests/todo-filter.spec.ts

• Gap identified: The All/Active/Completed filter buttons, "Clear completed" action, and empty state messages had no tests.
• User behavior: A user switches between filter views to see only active or completed todos, clears all completed todos in bulk (verifying the button disables when none remain), and sees contextual empty-state messages when a filter yields no results.

3. Todo Persistence — tests/todo-persistence.spec.ts

• Gap identified: localStorage-based persistence of todos across page reloads was untested.
• User behavior: A user adds and toggles todos, reloads the page, and confirms all changes survived the reload. After deleting a todo and reloading again, the deletion is confirmed persistent.

Test files

• tests/todo-crud.spec.ts (new)
• tests/todo-filter.spec.ts (new)
• tests/todo-persistence.spec.ts (new)
• tests/specs/test-plan.md (new)

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