Add first-class current/active workspace support

[nfebe]

Problem

The package exposes switchWorkspace(), currentWorkspace(), and currentOrDefaultWorkspace() on the HasWorkspaces trait, plus a switchTo controller action. But:

1. No storage — currentWorkspace() only works if the host app implements getCurrentWorkspaceId()/setCurrentWorkspaceId(). Every consumer reinvents this.
2. No read endpoint — there's no GET /workspaces/current to fetch the active workspace, so each app maintains a bespoke controller for it.

What we do today (workaround)

In the host app we back the hooks with whilesmart/eloquent-model-configuration, storing current_workspace_id as a user config value, and we keep a one-method app controller just to expose the current workspace (id, name, currency) to the frontend.

Proposed

• Provide a default storage implementation for current workspace (optionally via eloquent-model-configuration when present, else a current_workspace_id column / cache), so apps don't implement the hooks themselves.
• Add a GET /workspaces/current endpoint returning the current-or-default workspace, so the bespoke app controller can be dropped.

Tracking the app-side glue against this issue.