## What
The "new since last visit" highlight (#609) stored its marker in
`localStorage` (`transactions-last-visit`), so it was **per-device**:
each device kept its own last-visit timestamp and re-flagged the same
rows. This moves the marker to a per-user server column so a single
"last visit" is shared across all your devices.
## How
- New nullable `transactions_last_visited_at` column on `users` (same
pattern as `last_active_at` / `paywall_seen_at`).
- `TransactionController@index` reads the stored marker, renders the
list with the **old** value (`lastVisitAt` prop), then advances it
forward to the newest `created_at` it served.
- Frontend drops the `localStorage` read/write and just freezes the
`lastVisitAt` prop at mount; `isNewSince` per-row logic is unchanged.
## Why newest-served `created_at` and not `now()`
Advancing to `now()` would mark a back-dated synced row (old
`transaction_date`, lands on a later page that wasn't served) as seen,
so it would never be highlighted — the exact "hiding" failure the
per-row design avoids. Advancing only to what was actually served keeps
the feature's "err on showing, never hiding" stance. The marker only
ever moves forward.
## Notes
- Same filter caveat as before: opening the list with a filter applied
advances the marker using the filtered payload. Carried over from the
original `localStorage` behavior; not a regression.
- Removed the now-dead `loadLastVisit` / `saveLastVisit` /
`newestCreatedAt` helpers and their tests.
## Tests
- `NewTransactionsMarkerTest` (feature): null marker on first visit +
stores newest served; later visit sees previous marker + advances
forward; marker never moves backward.
- `new-transactions.test.ts` (`isNewSince`), Pint, ESLint, Prettier all
pass.
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