test: add browser end-to-end coverage for the documented ASEAN task

[elecpapaya]

Problem

Unit and structure tests do not prove that the public task path remains usable after layout, URL-state, or data changes.

Scope

Automate the documented ASEAN flow: select 2023 same-period data, ASEAN, Viet Nam, inspect intelligence/products/quality, restore a shared URL, and export evidence.

Acceptance criteria

• The test runs against validated synthetic fixtures without provider network calls.
• It checks visible period/provider labels and selected-country state on every relevant tab.
• Shared URL restoration and one CSV/JSON export are asserted.
• At least one desktop and one narrow viewport run in CI.
• Failures save a screenshot or trace artifact for diagnosis.

Out of scope

Replacing the three-person external usability study in issue #3.