Replace seenByTenant boolean with seenByTenantAt timestamp field

Update location tracking to record when tenant views a location rather than just whether they've seen it. This provides better audit trail and enables future features like viewing history.

Changes:
- Convert seenByTenant (boolean) to seenByTenantAt (Date) in database schema
- Update setSeenByTenantAt action to store timestamp instead of boolean flag
- Modify LocationCard UI to display when location was seen by tenant
- Update all references across locationActions, monthActions, and view components
- Remove unused imports from ViewLocationCard

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/ui/ViewLocationCard.tsx

@@ -1,7 +1,7 @@
 'use client';
 
-import { FC, useEffect, useMemo, useState } from "react";
-import { BillAttachment, BilledTo, BillingLocation, UserSettings } from "../lib/db-types";
+import { FC, useMemo, useState } from "react";
+import { BilledTo, BillingLocation, UserSettings } from "../lib/db-types";
 import { formatYearMonth } from "../lib/format";
 import { formatCurrency, formatIban } from "../lib/formatStrings";
 import { useTranslations } from "next-intl";