fix: keep the flash map intact in the put_toast sync path

[churcho]

When put_toast(socket, :success, "Saved") is followed by put_flash(socket, :error, "Check the form") before the client clears the success flash, the error flash does not render. The sync branch stores the whole assigns map under :f because the nested put_in expression returns the root map.

Use Map.put(assigns[:f], sync_toast_kind, nil) to update the flash map. The error flash now renders. The success message remains a stream toast, and its duplicate flash stays hidden. The tests also check the exact component flash map and one clear-flash event per matching update.

The change touches only lib/live_toast/live_component.ex, demo/test/demo_web/live/send_toast_test.exs, and CHANGELOG.md. Stream insertion, toast counts, dismissal, the public API, and JavaScript remain unchanged.

Check	Result
Regression file before the fix, on the implementation from main at 1ef5300	16/18 passed; the two new tests failed; exit 2
cd demo && mix test test/demo_web/live/send_toast_test.exs after the fix	18 passed; exit 0
cd demo && mix test	32 passed; exit 0
mix format --check-formatted	Exit 0
mix credo --strict at repository root	Exit 0
cd demo && mix credo --strict test/demo_web/live/send_toast_test.exs	Exit 0
git diff --check	Exit 0

Before the fix, the regression output included:

test LiveToast.put_toast/4 renders another flash while the synced flash awaits client clearing
Expected truthy, got false
code: assert has_element?(view, "#flash-error", "Check the form")

test LiveToast.put_toast/4 keeps the component flash map flat when removing a synced flash
Assertion with == failed
code: assert socket.assigns.f == %{"success" => nil, "error" => "Check the form"}

Result: 16/18 passed
Failed: 2 tests

After the fix:

Result: 18 passed

Full suite:

Result: 32 passed