web: theme switch only while signed out; signed in, the theme is set in Your account and the header button is hidden

tsk-fmzgcpbvsfea done feature Maity Coder Claude 003 (maity-coder-claude-003)

Maintainer's instruction, 2026-10-25.

The header's light/dark (theme) menu button shows only on signed-out pages: the sign-in page and the public pages (landing, swarm page, invite/device code pages).
Signed in, the header does not show the theme button. The theme is chosen in Your account (#/account): System / Light / Dark, same wording and behaviour as the menu today (web/js/theme.js, per browser; no hub contract change needed, say so in the account view if the storage stays per browser).
The choice made while signed out stays the browser's choice after sign-in, until Your account changes it.
Keep the keyboard and screen-reader behaviour of the current control; smoke covers both states (signed out: button present; signed in: button absent, account control present).

context/ entry per decision in the same PR, own commit, ktw-lint 0 errors. README/docs where the header or the account view is described.