Fix issue Context navigation, edit safety, and sidebar sizing

Problem

The issue-specific Context page does not give Context its own breadcrumb level, while its separate header back button creates a second, awkward return control. The context-item sidebar also stretches to the full height of the selected context pane instead of sizing to its own contents.

Linked project-wide context can currently be edited from an issue-specific page. A project context page may be shared by several issues, so editing it while focused on one issue makes the scope and impact easy to misunderstand.

Desired behavior

The issue Context manager should preserve a clear hierarchy back to the issue, use the breadcrumb as the return path, keep the page list compact, and make linked project context read-only. Project-wide context remains editable from the project Context tab; issue-scoped context remains editable from the issue Context manager.

Acceptance criteria

• Render the current issue as a navigable breadcrumb that returns to the issue detail page.
• Render Context as the terminal/current breadcrumb beneath the issue hierarchy.
• Remove the standalone back button from the issue Context page header.
• Keep the context-item sidebar content-height; it must not stretch to match a taller selected context document pane.
• Do not show edit/save controls for project-scoped context when it is viewed through an issue.
• Reject or remove issue-page mutation paths for project-scoped context, while preserving editing of issue-scoped context.
• Keep project-scoped context editable from the project Context tab.
• Add render and route tests for breadcrumb navigation, the absent header back button, sidebar sizing, read-only project context, and editable issue-only context.
• Preserve responsive layout, keyboard navigation, and dark-mode states.