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/proxy.ts

@@ -6,6 +6,7 @@ import {
   RETIRED_ROUTE_STATUS,
 } from "@/lib/http/retired-routes";
 import { setBaselineSecurityHeaders } from "@/lib/http/proxy-bypass-routes";
+import { DEMO_REFUSAL_BODY } from "@/lib/demo-refusal";
 
 /**
  * Paths that do NOT require a session cookie (public pages + external webhooks).
@@ -361,14 +362,7 @@ export function proxy(request: NextRequest) {
       )
     ) {
       return applyBaselineSecurityHeaders(
-        NextResponse.json(
-          {
-            data: null,
-            error: "Demo mode: modifications are disabled",
-            meta: { demo: true },
-          },
-          { status: 403 },
-        ),
+        NextResponse.json(DEMO_REFUSAL_BODY, { status: 403 }),
       );
     }
   }