Flip every user-facing read (dashboard, cashflow, transactions, budgets,
accounts, settings, analytics, sync, shared props) from user-scoped to
active-space-scoped via explicit $space->… relations, and re-key the category
tree, spending, and budget-assignment services by space_id. Authorization moves
from creator-ownership to space membership.
Add the visible spaces layer behind a Pennant 'spaces' flag: a sidebar space
switcher, a settings page to create/rename/delete/switch spaces, per-space
offline-cache flushing, and the currentSpace/spaces shared props. Data creation
is unchanged — the BelongsToSpace trait already stamps space_id from the active
space. Isolation tests prove no cross-space leakage.
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