feat(shell): 30/55/15 layout, formatted activity log, instant resume, stop confirmation

Left panels 30vw / central 55% / right rail 15vw, minimal padding. Model-activity
log and documents render via .thot-prose; decisions become hairline rows with
semantic type chips. Resume switches the view optimistically (instant feedback).
The Stop button now asks for confirmation before interrupting.

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

frontend/src/shell/StopConfirmDialog.tsx

@@ -0,0 +1,25 @@
+import { Dialog, DialogContent, DialogHeader, DialogTitle, DialogDescription, DialogFooter } from "../components/ui/dialog";
+import { Button } from "../components/ui/button";
+
+interface Props {
+  open: boolean;
+  onOpenChange: (open: boolean) => void;
+  onConfirm: () => void;
+}
+
+export function StopConfirmDialog({ open, onOpenChange, onConfirm }: Props) {
+  return (
+    <Dialog open={open} onOpenChange={onOpenChange}>
+      <DialogContent>
+        <DialogHeader><DialogTitle>Stop the session?</DialogTitle></DialogHeader>
+        <DialogDescription>
+          The running process will be interrupted. Progress so far is saved, and you can resume the session later.
+        </DialogDescription>
+        <DialogFooter>
+          <Button type="button" variant="outline" size="sm" onClick={() => onOpenChange(false)}>Cancel</Button>
+          <Button type="button" variant="destructive" size="sm" onClick={() => { onConfirm(); onOpenChange(false); }}>Stop &amp; save</Button>
+        </DialogFooter>
+      </DialogContent>
+    </Dialog>
+  );
+}