feat(coach): add a full-page conversation mode with a maximize control

Extract the Coach chat surface (header actions, message thread,
composer, provenance rails, settings, mobile trays) into a shared
<CoachConversation> component so the drawer and a new full-page route
render the exact same implementation — no forked chat logic. The drawer
keeps only its <Sheet> chrome (close button + maximize control); the
page supplies a minimize-back-to-drawer control.

The maximize control in the drawer header navigates to /insights/coach
and closes the drawer (aborting any in-flight stream); the page's
minimize control routes back to /insights and reopens the drawer through
the shared launch context. Both surfaces preserve incremental streaming,
collapsible history + provenance, the single composer disclaimer, the
auto-grow composer, refusal / budget handling, and the aiMode gating —
the full page redirects to /insights when the operator flag is off or
the user has opted out.

Lift useResettableValue / nextResettableValue into their own module so
the shared surface seeds its composer from the same controlled-prefill
contract without importing the drawer shell. Register the new route in
the coach feature-flag and per-user-disable gate-site invariants, and
add the maximize / minimize labels across all six locales.

src/app/insights/coach/page.tsx

@@ -0,0 +1,88 @@
+"use client";
+
+import { useEffect } from "react";
+import { useRouter } from "next/navigation";
+import { Minimize2 } from "lucide-react";
+
+import { Button } from "@/components/ui/button";
+import { CoachConversation } from "@/components/insights/coach-panel/coach-conversation";
+import { useTranslations } from "@/lib/i18n/context";
+import { useCoachLaunch } from "@/lib/insights/coach-launch-context";
+import { useFeatureFlags } from "@/hooks/use-feature-flags";
+import { useDisableCoach } from "@/hooks/use-disable-coach";
+
+/**
+ * v1.12.0 (Coach v2 #6) — full-page Coach conversation.
+ *
+ * Renders the exact same `<CoachConversation>` surface the drawer uses
+ * (history + streaming thread + composer + provenance + settings) at
+ * full width and height. There is no forked chat logic — the page is a
+ * thin chrome wrapper that supplies a "minimize back to drawer"
+ * control where the drawer supplies "maximize" + "close".
+ *
+ * The route lives inside `app/insights/layout.tsx` (sticky tab strip +
+ * mobile FAB) and the authenticated `<AuthShell>` (which provides the
+ * `<CoachLaunchProvider>` the minimize control reopens the drawer
+ * through), so the page inherits the same Coach feature gating as every
+ * other Insights surface. When the operator's master flag is off or the
+ * user opted out, the page redirects back to `/insights` rather than
+ * painting a dead chat shell.
+ */
+export default function CoachPage() {
+  const { t } = useTranslations();
+  const router = useRouter();
+  const launch = useCoachLaunch();
+  const flags = useFeatureFlags();
+  const disableCoach = useDisableCoach();
+
+  const coachUnavailable = !flags.coach || disableCoach;
+
+  // Coach surface gating mirrors `<CoachLaunchButton>` / `<LayoutCoachFab>`:
+  // operator master flag OR per-user opt-out hides the Coach entirely.
+  // Send a direct navigator back to the Insights mother page so the
+  // route is never a dead-end.
+  useEffect(() => {
+    if (coachUnavailable) {
+      router.replace("/insights");
+    }
+  }, [coachUnavailable, router]);
+
+  if (coachUnavailable) return null;
+
+  const handleMinimize = () => {
+    // Hand the conversation back to the drawer overlay: route to the
+    // Insights mother page and reopen the drawer through the shared
+    // launch context.
+    router.push("/insights");
+    launch?.askCoach(null);
+  };
+
+  return (
+    <div
+      data-slot="coach-page"
+      className="flex h-[calc(100dvh-13rem)] min-h-[28rem] flex-col overflow-hidden rounded-2xl border"
+    >
+      <CoachConversation
+        surface="page"
+        autoFocusComposer
+        renderTitle={(title) => (
+          <h1 className="min-w-0 truncate text-sm font-semibold">{title}</h1>
+        )}
+        leadingHeaderActions={
+          <Button
+            type="button"
+            variant="ghost"
+            size="icon"
+            onClick={handleMinimize}
+            data-slot="coach-page-minimize"
+            aria-label={t("insights.coach.minimizeAriaLabel")}
+            title={t("insights.coach.minimizeAriaLabel")}
+            className="text-muted-foreground hover:text-foreground size-11 shrink-0"
+          >
+            <Minimize2 className="size-4" aria-hidden="true" />
+          </Button>
+        }
+      />
+    </div>
+  );
+}