feat(mileage): surface Körjournal in the nav behind a settings toggle (#1540)

* feat(mileage): surface Körjournal in the nav behind a settings toggle

The /mileage page shipped hidden: the route works but no nav row points at
it. Add company_settings.mileage_enabled (mirroring dimensions_enabled) with
a switch in Fönster -> Bokföring, and show the Arbeta nav row when the toggle
is on OR the company already has mileage_trips rows, the same hybrid gate as
webshop orders, so trips created via API/MCP can never become invisible
underlag. UI visibility only, never load-bearing for correctness.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(migrations): move mileage_enabled migration after already-applied 20260812153208

origin/main merged in 20260812153208 which prod has already applied; a new
file sorting before it risks an out-of-order db push abort.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

tests/helpers.ts

@@ -622,6 +622,7 @@ export function makeCompanySettings(
     reminder_fee_amount: 60,
     reminder_interest_rate_override: null,
     dimensions_enabled: false,
+    mileage_enabled: false,
     preferred_payment_format: 'pain001',
     salary_pay_day: 25,
     salary_default_bank: null,