Fix iOS autofill of the sign-in code

[rzueger]

Problem

On iOS, when the 6-digit email sign-in code is offered above the keyboard
("From email: 123456"), tapping it filled only the first digit.
Reported by an LSZO customer; reproduced on lszo-test.web.app with
Brave + Apple Mail.

Cause

The code field is six separate <input maxLength={1}> boxes, and only
the first was marked autoComplete="one-time-code" (the rest were
off). iOS Security-Code AutoFill inserts the whole code into one
field; maxLength={1} truncated it to one digit and handleChange
kept only the last character, discarding the other five.

Fix (src/components/LoginPage/OtpCodeForm.tsx)

• Remove the per-box maxLength={1} (single-char display is already
  enforced by the controlled value), so the full autofilled code
  reaches onChange.
• Mark every box autoComplete="one-time-code".
• Spread a multi-character value across the boxes — the same
  distribution logic the paste handler used, factored into a shared
  fillFrom(startIndex, digits) helper — and auto-submit when complete.

Keeps the segmented UI, the data-cy handles, and all existing keyboard
/ paste behaviour unchanged.

Notes / scope

• Applies to all iOS browsers (Brave, Chrome, etc. are all WebKit).
• Codes arriving in the Gmail app are unaffected by this or any code
  change — iOS only auto-suggests codes from the Apple Mail app.
  Those users continue to copy-paste, which already works.

Verification

• npm run typecheck clean; full Jest suite green (2284 tests).
• OtpCodeForm.spec.tsx extended with an autofill group: a single
  change carrying the whole code into the first field fills all six boxes
  and auto-submits.
• npm run build --project=lszm compiles.
• On-device final check pending after deploy to lszo-test (cannot be
  tested in-repo; the jest autofill test is the local proxy).