fix(layout): normalise auth-shell container width on max-w-screen-xl

Pre-v1.4.33 the dashboard shell capped at max-w-[76.8rem] (1228 px)
while settings/admin shells used max-w-screen-xl (1280 px) and the
bugreport route capped at max-w-6xl (1152 px). Route switches between
those surfaces produced a 52-128 px lateral shift on a 1440 px laptop
which the user perceives as the page "wobbling".

- src/components/layout/auth-shell.tsx: max-w-[76.8rem] -> max-w-screen-xl
- src/app/bugreport/page.tsx: drop the inner max-w-6xl cap so the shell
  alone owns the content frame.

Settings/admin shells already use max-w-screen-xl; no change needed.

Refs .planning/round-v1433-audit-polish.md §4.2 + Win 1.

src/app/bugreport/page.tsx

@@ -99,7 +99,7 @@ export default function BugReportPage() {
 
   if (!isAuthenticated) {
     return (
-      <div className="mx-auto w-full max-w-6xl space-y-6">
+      <div className="w-full space-y-6">
         <h1 className="text-2xl font-bold tracking-tight">
           {t("bugreport.title")}
         </h1>
@@ -112,7 +112,7 @@ export default function BugReportPage() {
 
   if (status && status.enabled === false) {
     return (
-      <div className="mx-auto w-full max-w-6xl space-y-6">
+      <div className="w-full space-y-6">
         <div>
           <h1 className="text-2xl font-bold tracking-tight">
             {t("bugreport.title")}