Add confirmation flow for clearing completed todos

[multiplex55]

Motivation

• Prevent immediate destructive mutation of the todo list when the user clicks "Clear Completed" by requiring explicit confirmation.
• Keep the UI state and persisted todo.json in sync after a confirmed clear.

Description

• Add a pending_clear_confirm: bool field to TodoDialog and initialize it in open to track awaiting confirmation.
• Replace the immediate entries.retain(...) behavior with a local confirmation UI rendered inside the todo dialog and a helper confirm_clear_completed(&mut self, app: &mut LauncherApp, confirmed: bool) that only mutates and persists entries when confirmed.
• Wire the dialog buttons so clicking "Clear Completed" sets pending_clear_confirm = true and the dialog shows a small confirm/cancel row; clicking confirm invokes the helper to remove done items and call save(...) only if the list changed.
• Add unit tests clear_completed_requires_confirmation and clear_completed_after_confirmation_saves to cover the unconfirmed/confirmed flows and persistence via load_todos/save_todos.

Testing

• Added unit tests in src/gui/todo_dialog.rs: clear_completed_requires_confirmation and clear_completed_after_confirmation_saves which assert no removal before confirmation and proper removal + save after confirmation.
• No automated test run was performed as part of this change (tests were added but not executed in CI/local run).

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Codex Task