fix(finance): default the journal currency when settings are absent

A fresh install dies creating the first airline:

    App\Models\Airline::initJournal(): Argument #1 ($currency_code)
    must be of type string, null given

setting() returns its $default for a key it cannot read -- retrieve()
throws SettingNotFound and the helper swallows it -- and the created
hook passed no default, so it handed null to a string parameter. The
'USD' default on initJournal() cannot help: an explicit argument always
beats a declared default, even when that argument is null.

The installer creates the airline and user in the same step, and the
settings seed is gated on a separate wizard step, so units.currency is
not guaranteed to exist by then. Every other reader of this setting
already names the fallback -- Money.php:59,74 and
PirepFinanceService.php:57,61 -- so this is the odd one out.

Airline and User are the only models using the trait; both are covered.
The suite could not have caught this because tests/Pest.php seeds
SettingsSeeder before every test, making an empty settings table
unreachable, so the new tests clear it explicitly.

app/Traits/JournalTrait.php

@@ -14,7 +14,13 @@ trait JournalTrait
     public static function bootJournalTrait(): void
     {
         static::created(function ($model): void {
-            $model->initJournal(setting('units.currency'));
+            // The default on initJournal() cannot cover this: an explicit
+            // argument always beats a declared default, and setting() returns
+            // null for a key it cannot read. During install the first airline
+            // is created before units.currency necessarily exists, so passing
+            // the bare lookup hands a null to a string parameter. Every other
+            // caller of this setting already names the fallback.
+            $model->initJournal(setting('units.currency', 'USD'));
         });
     }
 

tests/Unit/JournalTraitTest.php

@@ -3,6 +3,8 @@
 declare(strict_types=1);
 
 use App\Models\Airline;
+use App\Models\Setting;
+use App\Models\User;
 
 /*
  * Regression coverage for JournalTrait::initJournal().
@@ -27,3 +29,44 @@ test('initJournal populates the journal relation on the same instance', function
     // same journal a fresh reload from the database resolves to.
     expect($airline->journal->id)->toBe($airline->fresh()->journal->id);
 });
+
+/*
+ * The `created` hook reads units.currency, and setting() returns null for a key
+ * it cannot read -- the settings table is empty during install, and the first
+ * airline is created there. initJournal() declares a 'USD' default, but a
+ * default only applies when the argument is omitted; passing the bare lookup
+ * hands an explicit null to a string parameter and the install dies with
+ * "Argument #1 ($currency_code) must be of type string, null given".
+ *
+ * tests/Pest.php seeds SettingsSeeder before every test, so the empty-settings
+ * state these two cover is otherwise unreachable from the suite.
+ */
+test('a model with a journal can be created before units.currency is seeded', function (): void {
+    Setting::query()->delete();
+
+    $airline = Airline::factory()->create();
+
+    expect($airline->journal)->not->toBeNull()
+        ->and($airline->journal->currency)->toBe('USD');
+});
+
+test('users are also covered, not just airlines', function (): void {
+    Setting::query()->delete();
+
+    // Airline and User are the only two models using the trait, and the
+    // installer creates both back to back.
+    $user = User::factory()->create();
+
+    expect($user->journal)->not->toBeNull()
+        ->and($user->journal->currency)->toBe('USD');
+});
+
+test('a seeded units.currency still wins over the fallback', function (): void {
+    // Settings are already seeded by tests/Pest.php, so this is the normal
+    // path: the fallback must not shadow a value that is actually present.
+    // Matched on `key` -- the primary key stores dots as underscores
+    // (`units_currency`), which SettingService normalises on lookup.
+    Setting::where('key', 'units.currency')->update(['value' => 'EUR']);
+
+    expect(Airline::factory()->create()->journal->currency)->toBe('EUR');
+});