Rebuild the Coach thinking row as one calm line

While a turn runs the Coach now shows a single row: a brain icon, the
current step in the answer's text size with a shimmer passing through it
and fading dots, and a chevron once there are steps to open. There is no
running clock any more, and the interim table preview with its sparkline
no longer renders under the row; interim tables stay in state until the
answer's own results show them.

When the answer is done the row reads "Thought process · N steps" with
proper plurals, and "Answer needed" while a clarifying question waits.
It never opens by itself, and a row opened during the run closes when
the turn ends. Opened, the steps form a vertical list with an icon per
kind of work on a thin connector line, and a reasoning round shows its
summary under its title.

The shimmer and the dots are a `text-shimmer` utility and a reusable
WaitingText component, both still under reduced motion.

The demo's write block now carries `meta.errorCode: "demo.readOnly"`,
listed in the error-code catalogue, and the Coach answers it with an
honest line instead of the provider-outage copy.

Removes the interim preview component, the elapsed-seconds and summary
helpers, and their message keys in all seven locales.

src/lib/openapi/error-codes.ts

@@ -118,6 +118,7 @@ export const ERROR_CODE_CATALOGUE: Readonly<Record<string, readonly string[]>> =
     ],
     "cycle-prefs": ["cycle-prefs.body.invalid_json", "cycle-prefs.invalid"],
     "disable-coach": ["disable-coach.body.invalid_json"],
+    demo: ["demo.readOnly"],
     documents: [
       "documents.chat.conversationNotFound",
       "documents.chat.invalid",