Shared Finances — share specific transactions/categories with other users

Allow a user to expose specific transactions (selected by tag, category, or other rule) to one or more other users, without giving them full account access.

Use case: "financial accountability" — letting a partner, accountability buddy, or advisor see a curated slice of activity (e.g., only dining-out spend, only transactions tagged #discretionary) while everything else stays private.

Granularity should be fine-grained: rule-based selection rather than all-or-nothing per account.