Frontend workspace switcher and management UI

Wires up the long-pending frontend half of the workspace feature.
Adds a sidebar workspace switcher that persists the active selection
to localStorage, scopes the projects list and new-project creation to
that workspace, and provides a /workspaces management page (list,
create, settings, members, danger-zone delete).

Workspace admins can edit name/description/instructions, set the
external-AI policy override and retention days. Owners can change
member roles, invite by email and delete non-Personal workspaces.

Also rebrands the sidebar wordmark from "Mike" to "PIP".