Add seenByTenant tracking feature

- Add seenByTenant field to BillingLocation interface
- Implement setSeenByTenant function to mark locations as viewed by tenant
  - Checks if flag is already set to avoid unnecessary DB updates
  - Includes TypeDoc documentation
- Update LocationViewPage to call setSeenByTenant when non-owner visits
- Add seenByTenant to fetchAllLocations projection
- Update LocationCard to show "seen by tenant" status indicator
  - Displays in "Monthly statement" fieldset with checkmark icon
  - Shows alongside monthly expense total
- Add localization strings for monthly statement and seen status

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

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

app/lib/actions/locationActions.ts

@@ -411,6 +411,7 @@ export const fetchAllLocations = withUser(async (user:AuthenticatedUser, year:nu
                     "yearMonth.year": 1,
                     "yearMonth.month": 1,
                     "bills": 1,
+                    "seenByTenant": 1,
                     // "bills.attachment": 0,
                     // "bills.notes": 0,
                     // "bills.barcodeImage": 1,
@@ -529,4 +530,36 @@ export const deleteLocationById = withUser(async (user:AuthenticatedUser, locati
         message: null,
         errors: undefined,
     };
-})
+})
+
+/**
+ * Sets the `seenByTenant` flag to true for a specific location.
+ *
+ * This function marks a location as viewed by the tenant. It first checks if the flag
+ * is already set to true to avoid unnecessary database updates.
+ *
+ * @param {string} locationID - The ID of the location to update
+ * @returns {Promise<void>}
+ *
+ * @example
+ * await setSeenByTenant("507f1f77bcf86cd799439011");
+ */
+export const setSeenByTenant = async (locationID: string): Promise<void> => {
+    const dbClient = await getDbClient();
+
+    // First check if the location exists and if seenByTenant is already true
+    const location = await dbClient.collection<BillingLocation>("lokacije")
+        .findOne({ _id: locationID });
+
+    // If location doesn't exist or seenByTenant is already true, no update needed
+    if (!location || location.seenByTenant === true) {
+        return;
+    }
+
+    // Update the location to mark it as seen by tenant
+    await dbClient.collection<BillingLocation>("lokacije")
+        .updateOne(
+            { _id: locationID },
+            { $set: { seenByTenant: true } }
+        );
+}