fix(chat): centre the empty-chat hero in the reading column

[Astro-Han]

Summary

The empty chat surface rendered its hero — the wordmark plus the time-of-day greeting — noticeably low, sitting against the composer rather than in the middle of the reading column.

The cause is in Astryx's ChatMessageList: it renders a flex: 1 spacer ahead of its rows so a short conversation rests on the composer instead of hanging from the top. That spacer is unconditional, so on the empty surface it split the column with the empty-state holder — which is itself flex: 1 and centres its content — and the hero ended up centred in the bottom half. Measured on main: hero centre 552px against a column centre of 380px, a 172px drop, plus the list's own 16px gap.

This collapses the spacer when the list's last row is an empty state, handing the whole column back to the holder so its centring is true to the surface. The rule is keyed on the empty-state content rather than on "no messages" because a one-message conversation must keep the push-to-bottom.

The accompanying check is a live metric rather than a static CSS read: the selector matches ChatMessageList's internal DOM, so a CSS contract could only prove the rule is written, never that it still matches after an Astryx bump.

Verification

• apps/desktop Playwright — full e2e/scroll-geometry.spec.ts, 6 passed, including the new the empty-chat hero centres in the reading column.
• The new test was confirmed to fail for the right reason: with the spacer left visible it reports the 172px offset and goes red.
• npm run lint, npm run format:check, node scripts/check-dead-css.mjs --check (the allowlist entry is for astryx-empty-state, which Astryx emits through themeProps at runtime and never appears as a className literal in Maka source).
• Live app on this branch, empty session: hero centre and message-list centre both read 380px. Conversation with rows: the spacer still computes display: block, so push-to-bottom is intact.