Fix TodoMVC toggle-all control in dioxus-native

[NexusAndromeda]

Summary

• Fixes the TodoMVC toggle all control not working in dioxus-native (TodoMVC Example #145).
• Replaces the hidden checkbox + label pattern with an explicit button and onclick handler.
• Adds a headless regression test via blitz-test-harness.

Test plan

• cargo test -p blitz-tests todomvc_toggle_all_has_visible_label_content --test todomvc_toggle_all
• Manual: cargo run -p todomvc --bin todomvc_native, create todos, click toggle-all to mark/unmark all