license-activation fixture

The license-key-activation e2e spec drives the real chrome-extension://<id>/options.html#upgrade surface, so no html host page is actually required for the in-page flow.

This file is shipped because the per-spec allowed_paths inventory in .lead/tasks/phase1-w4-t3.json lists it, and keeping the fixture as a documented placeholder avoids a follow-up "create empty file" task. The spec stubs the SW chrome.runtime.sendMessage({type:'license:activate'}) handler via addInitScript (see docs/e2e-license-key-activation.md) and asserts on chrome.storage.local.'mailshade.license' + the Senders section transitioning from read-only to editable.