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/components/ui/waiting-text.tsx

@@ -0,0 +1,50 @@
+import { cn } from "@/lib/utils";
+
+/** A trailing ellipsis, three dots or the single character. */
+const TRAILING_ELLIPSIS = /(?:\.\.\.|…)\s*$/u;
+
+/** The stagger of the three dots, one class each (no inline style). */
+const DOT_DELAYS = [
+  "[animation-delay:0s]",
+  "[animation-delay:0.2s]",
+  "[animation-delay:0.4s]",
+] as const;
+
+/**
+ * Text that is still waiting on something: a trailing "..." or "…" becomes
+ * three dots that fade in turn (`waiting-dot` in `globals.css`), so the line
+ * reads as alive without a spinner beside it. Text without a trailing
+ * ellipsis renders as it is. Under reduced motion the dots stand still.
+ *
+ * `textClassName` styles the words alone, never the dots: a text effect
+ * clipped to the glyphs (`text-shimmer`) does not reliably reach a child
+ * that animates its own opacity, so the dots keep the surrounding colour.
+ *
+ * The dots are ordinary full stops, so the text reads and copies as
+ * "Thinking...". A surface that announces its status elsewhere hides this
+ * text from assistive technology itself.
+ */
+export function WaitingText({
+  text,
+  textClassName,
+}: {
+  text: string;
+  textClassName?: string;
+}) {
+  const match = TRAILING_ELLIPSIS.exec(text);
+  const words = match ? text.slice(0, match.index) : text;
+  return (
+    <>
+      <span className={textClassName}>{words}</span>
+      {match ? (
+        <span data-slot="waiting-dots">
+          {DOT_DELAYS.map((delay) => (
+            <span key={delay} className={cn("waiting-dot", delay)}>
+              .
+            </span>
+          ))}
+        </span>
+      ) : null}
+    </>
+  );
+}