## 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`_
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