feat(shell): yellow active session, growing composer, phase title, refined activity log, arrow panel toggle

Fix six GUI defects, verified live against the real stack:

- NavSessions: mark the in-progress (active) session in yellow (warning
  token) — both its status dot and its row background.
- SteerInput: the composer is now an auto-growing textarea that wraps and
  grows vertically (caps at 160px, then scrolls); Enter sends, Shift+Enter
  inserts a newline.
- WorkflowBar: show a synthetic title of the current phase under the 8 dots
  from static EN/IT strings (no LLM); English is displayed to match the chrome.
- CentralStatus: reformat the 5-line system-message tail as a structured,
  monospace list with per-line markers and an emphasized last line.
- AppShell + CentralStatus: move the left Model-activity panel toggle off the
  working spinner (now a pure status indicator) onto a dedicated arrow button
  (→ opens, ← closes).
- gate: rename the phase-confirm button "Conferma e prosegui" → "Salva e
  procedi" (builders.js + its L1 test).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/shell/AppShell.tsx

@@ -11,7 +11,7 @@ import { DeleteConfirmDialog } from "./DeleteConfirmDialog";
 import { StopConfirmDialog } from "./StopConfirmDialog";
 import { SteerInput, ComposerFooter } from "./SteerInput";
 import { WorkflowBar } from "./WorkflowBar";
-import { Pencil } from "lucide-react";
+import { Pencil, ArrowLeft, ArrowRight } from "lucide-react";
 import { Button } from "../components/ui/button";
 import { Toaster } from "../components/ui/sonner";
 import { toast } from "sonner";
@@ -34,7 +34,7 @@ export function AppShell() {
   const { data: sessions = [] } = useQuery<SessionSummary[]>({
     queryKey: ["sessions"], queryFn: listSessions, refetchInterval: 10_000,
   });
-  const composerRef = useRef<HTMLInputElement>(null);
+  const composerRef = useRef<HTMLTextAreaElement>(null);
 
   const queryClient = useQueryClient();
   const [panelSession, setPanelSession] = useState<SessionSummary | null>(null);
@@ -194,7 +194,18 @@ export function AppShell() {
       {/* Conversation column */}
       <div className="flex min-w-0 flex-1 flex-col">
         {activeSessionId && (
-          <div className="shrink-0 border-b border-border/70 bg-background/80 px-6 py-2.5 backdrop-blur">
+          <div className="relative shrink-0 border-b border-border/70 bg-background/80 px-6 py-2.5 backdrop-blur">
+            {/* Arrow toggle for the left Model-activity panel: → opens it, ← closes it. */}
+            <button
+              type="button"
+              onClick={toggleActivity}
+              aria-expanded={showActivity}
+              aria-label={showActivity ? "Hide model activity" : "Show model activity"}
+              title={showActivity ? "Hide model activity" : "Show model activity"}
+              className="absolute left-4 top-1/2 grid size-7 -translate-y-1/2 place-items-center rounded-md border border-border text-muted-foreground transition-colors hover:bg-muted hover:text-foreground"
+            >
+              {showActivity ? <ArrowLeft className="size-4" /> : <ArrowRight className="size-4" />}
+            </button>
             <div className="mx-auto max-w-3xl">
               <WorkflowBar
                 finalized={finalized}
@@ -209,7 +220,7 @@ export function AppShell() {
           <div className="mx-auto flex max-w-3xl flex-col gap-6">
             {activeSessionId ? (
               <>
-                <CentralStatus working={working} onOpenActivity={toggleActivity} />
+                <CentralStatus working={working} />
                 <WidgetHost sessionId={activeSessionId} />
               </>
             ) : (