fix(ui): expand dashboard directly from its side-panel tab (#143579)

fix(ui): expand dashboard directly from its side-panel tab (#143579)

Expand Dashboard directly from its side-panel tab and expose an expand/restore control beside the independent close button. Preserve the original main/side arrangement, geometry, mounted widgets and chat state across restoration.

Keep keyboard activation, ordinary tab navigation, other side panels and narrow/bottom layouts intact. Update the MCP retained-runtime scenario to assert expansion and explicitly restore split without weakening identity, input or request-count checks.

Validated with focused unit tests, production-bundled Chromium scenarios, changed-file gates, fresh P2 autoreview, inspected before/expanded/restored captures, and exact-head CI.

Closes #143575.

Co-authored-by: fuller-stack-dev <263060202+fuller-stack-dev@users.noreply.github.com>