bug: login fails after switching theme (needs repro) #1345

Parent: #830 (chat leftovers, 9 Sep report). Needs repro. Verified against main @ 8e39053.

Report

A user could not log in after switching themes (dark/light).

What we know

• No fix has landed since. The only theme commit after the report is fix(sidebar): prevent theme toggle truncation #1123 (toggle label truncation).
• frontend/e2e/desktop/theme.electron.spec.ts (~L191-226) checks that the theme persists to /auth/login after relaunch, but never logs in afterwards, so this path is untested.

Steps to try

1. Fresh install on 3.13.0 (and on main). Log in, switch to dark, log out, then log in again.
2. Same, but switch the theme on the login screen itself, then log in.
3. Switch the theme, restart the app, then log in. Try macOS and Windows.
4. Record the network response of POST /api/auth/login and the console.

Acceptance

• Either a repro with steps, followed by a fix and an e2e that logs in after a theme switch, or "cannot reproduce on 3.13.0", with the e2e login-after-switch step added anyway and this issue closed.