[Stage 1.3] Extract Storybook and E2E into private workspace projects

[draedful]

Parent: #318

Goal

Make Storybook and E2E real consumers of @gravity-ui/graph instead of tools that bypass the package boundary through repository source imports.

Scope

• Move Storybook into a dedicated private workspace project.
• Move Playwright E2E setup, scenarios, and the current Page Objects into a dedicated private workspace project.
• Declare @gravity-ui/graph as a workspace dependency of both projects.
• Replace relative or aliased imports from graph source with public package entry points.
• Export ESchedulerPriority and any scheduler contract required by Page Objects from @gravity-ui/graph.
• Make GraphPageObject consume that neutral graph contract instead of importing the scheduler implementation.
• Keep publication of a standalone Page Object package out of this task; it remains a later package-extraction stage.

Acceptance criteria

• Storybook and E2E workspace manifests set private to true.
• Both projects build and run against @gravity-ui/graph package entry points.
• Neither project imports packages/graph/src or other graph implementation paths.
• GraphPageObject imports ESchedulerPriority through @gravity-ui/graph.
• Storybook build and the complete E2E suite pass.
• The packed @gravity-ui/graph consumer checks remain green.