Fixes a linked GP entity showing an empty Assets tab when its capital is
held under the linked account's other legal names (the eNAV often splits
one LLC across names).
- New GET /api/entities/{id}/asset-balances: household-aware, returns the
linked account's (and its linked names') capital balances per fund, plus
the linked account name for a clear empty state.
- Assets tab uses it; shows "linked to X but no balances on file" instead
of a blank table when the wrong account is linked.
- GP/mgmt Overview now surfaces the total linked balance across funds with
a "View by fund" link, so assets are visible without opening the tab.
Verified: 15/15 backend tests (incl. household case); frontend tsc + vite
build clean.
