fix(balances): allow saving a zero balance (#664)

## Why

A balance of exactly **0** could not be saved. Zero is a perfectly valid
balance for any account (an emptied wallet, a paid-off loan, a closed
position), so the app was rejecting legitimate input.

## Root cause

The shared `AmountInput` renders a numeric value of `0` as an **empty
string** (so the field shows the `0.00` placeholder instead of a literal
"0.00"). Several balance forms combined that with the native HTML
`required` attribute — which then rejected the *only* value that renders
empty: `0`. `required` on this input never guarded against a missing
value (the state is always numeric; empty == 0), it only ever blocked
zero.

The backend already accepted `0` (`required|integer`, and Laravel's
`required` treats integer `0` as present), so this was purely a frontend
constraint.

## Changes

- Drop `required` from the balance `AmountInput` in the **Update
balance** dialog and the **Balance history** edit modal.
- Remove the explicit `balanceInCents === 0` guard (and `required`) in
the onboarding **Set balance** step.
- Extend the same fix to the CSV-import **reference balance** field
(identical root cause; the compute path already gates on
`referenceBalance !== null`, so an explicit `0` is a valid anchor).
- Remove the now-orphaned `"Please enter a balance"` translation from
`es.json` / `fr.json`.
- Add feature tests asserting a zero balance can be stored and set as
the current balance.

Transaction and budget amount inputs keep `required` — `0` is not a
meaningful value there, so they are intentionally untouched.

## QA

- **Onboarding balance step**: saving with an empty/zero field now
advances instead of showing "Please enter a balance". ✅
- **Update balance dialog**: set €1,000.00, then overwrote it with €0.00
→ dialog closed and the record persisted. Verified in the DB
(`account_balances.balance = 0`). ✅
- Backend contract locked by new tests in
`AccountBalanceControllerTest`.

## Demo

<!-- PLACEHOLDER: drag the video here -->

_Video to attach: `~/Downloads/allow-zero-balances-demo.mp4`_

lang/es.json

@@ -1240,7 +1240,6 @@
     "Pink": "Rosa",
     "Platform": "Plataforma",
     "Please describe the problem and what you were doing when it happened. Keep the details below — they help us debug.": "Describe el problema y qué estabas haciendo cuando ocurrió. Conserva los detalles de abajo: nos ayudan a depurar.",
-    "Please enter a balance": "Por favor, ingresa un balance",
     "Please enter a bank name.": "Por favor, ingresa un nombre de banco.",
     "Please enter an account name.": "Por favor, ingresa un nombre de cuenta.",
     "Please enter your new password below": "Por favor ingresa tu nueva contraseña a continuación",

lang/fr.json

@@ -1199,7 +1199,6 @@
     "Pink": "Rose",
     "Platform": "Plate-forme",
     "Please describe the problem and what you were doing when it happened. Keep the details below — they help us debug.": "Décrivez le problème et ce que vous faisiez lorsqu'il s'est produit. Conservez les détails ci-dessous : ils nous aident à déboguer.",
-    "Please enter a balance": "Veuillez saisir un solde",
     "Please enter a bank name.": "Veuillez saisir un nom de banque.",
     "Please enter an account name.": "Veuillez saisir un nom de compte.",
     "Please enter your new password below": "Veuillez entrer votre nouveau mot de passe ci-dessous",

resources/js/components/accounts/balances-modal.tsx

@@ -441,7 +441,6 @@ export function BalancesModal({
                                 value={editAmount}
                                 onChange={setEditAmount}
                                 currencyCode={account.currency_code}
-                                required
                             />
                         </div>
 

resources/js/components/accounts/update-balance-dialog.tsx

@@ -199,7 +199,6 @@ export function UpdateBalanceDialog({
                                 value={balance}
                                 onChange={setBalance}
                                 currencyCode={account.currency_code}
-                                required
                             />
                         )}
                     </div>

resources/js/components/onboarding/step-import-balances.tsx

@@ -24,11 +24,6 @@ export function StepImportBalances({
         e.preventDefault();
         setError(null);
 
-        if (balanceInCents === 0) {
-            setError(__('Please enter a balance'));
-            return;
-        }
-
         setIsSubmitting(true);
 
         try {
@@ -100,7 +95,6 @@ export function StepImportBalances({
                         onChange={setBalanceInCents}
                         currencyCode={account?.currencyCode || 'USD'}
                         disabled={isSubmitting}
-                        required
                     />
                 </div>
 

resources/js/components/transactions/import-step-mapping.tsx

@@ -428,7 +428,6 @@ export function ImportStepMapping({
                                         value={referenceBalance ?? 0}
                                         onChange={onReferenceBalanceChange}
                                         currencyCode={currencyCode}
-                                        required
                                     />
                                     {referenceBalancePrefilled && (
                                         <p className="text-xs text-muted-foreground">

tests/Feature/AccountBalanceControllerTest.php

@@ -280,6 +280,47 @@ it('can store balance without invested_amount', function () {
     ]);
 });
 
+it('can store a zero balance', function () {
+    $user = User::factory()->create();
+    $account = Account::factory()->for($user)->create();
+
+    $response = $this->actingAs($user)->postJson("/api/accounts/{$account->id}/balances", [
+        'balance' => 0,
+        'balance_date' => now()->subDays(5)->toDateString(),
+    ]);
+
+    $response->assertStatus(201)
+        ->assertJsonPath('data.balance', 0);
+
+    $this->assertDatabaseHas('account_balances', [
+        'account_id' => $account->id,
+        'balance' => 0,
+    ]);
+});
+
+it('can update the current balance to zero', function () {
+    $user = User::factory()->create();
+    $account = Account::factory()->for($user)->create();
+
+    AccountBalance::factory()->for($account)->create([
+        'balance_date' => now()->toDateString(),
+        'balance' => 100000,
+    ]);
+
+    $response = $this->actingAs($user)->putJson("/api/accounts/{$account->id}/balance/current", [
+        'balance' => 0,
+    ]);
+
+    $response->assertSuccessful()
+        ->assertJsonPath('data.balance', 0);
+
+    $this->assertDatabaseHas('account_balances', [
+        'account_id' => $account->id,
+        'balance_date' => now()->toDateString(),
+        'balance' => 0,
+    ]);
+});
+
 it('validates store balance must be an integer', function () {
     $user = User::factory()->create();
     $account = Account::factory()->for($user)->create();