feat(frontend): rotating activity icon + 5-line expandable activity panel

- CentralStatus: replace the red pulsing dot at the start of the "working" line
  with the rotating WorkingSpinner, now a button that opens the model-activity
  panel (onOpenActivity). WorkingSpinner is extracted to its own module (was
  local to AppShell).
- ModelActivityPanel: show the last 5 lines of the model-stream tail (refreshing
  as the stream grows) with an expand toggle to the full stream, replacing the
  unbounded full transcript.
- AppShell: drop the separate spinner button (the inline icon is now the single
  trigger) and the local WorkingSpinner def.
- Remove the now-orphaned Transcript.tsx (its only consumer was the panel).

TDD: CentralStatus + ModelActivityPanel tests RED->GREEN; frontend suite 87/87;
tsc clean. Live visual verification pending (to do with the B/C frontend pass).

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

frontend/src/shell/CentralStatus.tsx

@@ -1,5 +1,6 @@
 import { useEffect, useReducer, useState } from "react";
 import { useSessionStore } from "../store/sessionStore";
+import { WorkingSpinner } from "./WorkingSpinner";
 
 /** Last few words of a string — a compact "what the model is doing now" tail. */
 function lastWords(s: string, n = 8): string {
@@ -11,7 +12,13 @@ function lastWords(s: string, n = 8): string {
  *  (elapsed time + a short tail of the model stream) while the model is busy, and the
  *  gate's curated messages for the current step. The full verbose stream lives in the
  *  left Model-activity panel. */
-export function CentralStatus({ working }: { working: boolean }) {
+export function CentralStatus({
+  working,
+  onOpenActivity,
+}: {
+  working: boolean;
+  onOpenActivity?: () => void;
+}) {
   const lastUserEntry = useSessionStore((s) => s.lastUserEntry);
   const stepMessages = useSessionStore((s) => s.stepMessages);
   const transcript = useSessionStore((s) => s.transcript);
@@ -49,7 +56,15 @@ export function CentralStatus({ working }: { working: boolean }) {
 
       {working && (
         <div className="flex min-w-0 items-center gap-2 text-sm text-muted-foreground">
-          <span className="size-1.5 shrink-0 animate-pulse rounded-full bg-primary" aria-hidden />
+          <button
+            type="button"
+            onClick={onOpenActivity}
+            aria-label="Show model activity"
+            title="Model activity"
+            className="grid size-5 shrink-0 place-items-center rounded text-primary transition-colors hover:bg-muted"
+          >
+            <WorkingSpinner spinning className="size-3.5" />
+          </button>
           <span className="shrink-0 tabular-nums">{elapsed}s</span>
           {tail && <span className="min-w-0 truncate text-muted-foreground/70">· {tail}</span>}
           <span className="sr-only">ThothII is working</span>