Prefactor: split the workspaces store into server state and selection state

[ExtraToast]

What to build

Prefactor: the workspaces store is split along the seam that already exists inside it, separating fetched server state from client-only selection and filter state.

At roughly four hundred lines it is by far the largest store in the application, and it mixes two things with completely different destinations — one half becomes query cache, the other half stays Pinia. Splitting first makes the later conversion a small, reviewable change instead of an unreviewable one.

Make the change easy, then make the easy change.

Acceptance criteria

• Fetched state and client-only state are separated
• No behaviour changes and no screen changes
• The split is visible in the tests, so each half is exercised independently
• The client-only half is recognisable as something that will stay

Blocked by

None — can start immediately.