[P0] Realign end-to-end tests with the current UI contract

[nkanf-dev]

Summary

Rewrite the E2E suite so that it validates the current application shell and real user flows instead of historical selectors and outdated copy.

Why

The current Playwright tests assert against elements that do not exist in the shipped UI, which means the suite cannot act as a release gate.

Current evidence

• src/tests/e2e/app-load.test.ts expects .logo.
• src/tests/e2e/workflow.test.ts expects selectors such as Select PDF to Start, .document-info, and .quick-flip-overlay that do not match the current app.

Scope

• Update selectors to use stable roles, accessible names, and resilient test ids where appropriate.
• Cover the current golden path: open app, import PDF, navigate, open quick flip, hold a page, open a comparison window.
• Remove brittle assumptions tied to outdated markup.
• Keep test fixtures minimal and realistic.

Acceptance criteria

• The app shell test passes against the current UI.
• The main workflow test passes end to end on CI.
• Tests use stable selectors rather than styling classes whenever possible.

Out of scope

• Expanding E2E coverage to every edge case.