style(layout): unify top-of-page padding between AuthShell, Settings, and Admin

Settings and Admin pages render inside AuthShell, whose inner main
already applies `mx-auto max-w-[76.8rem] px-4 py-6 md:px-6`. Both
shells then re-applied `px-4 py-6 md:px-6 md:py-8` on their own
container, which produced visibly more top/bottom whitespace on
/settings/* and /admin/* than on /, /measurements, /medications,
/mood, /insights, /coach, and /notifications.

Drop the redundant padding from SettingsShell and AdminShell so the
visual top-of-page baseline matches the rest of the app. The
`mx-auto w-full max-w-screen-xl` wrapper stays for the wider
content column that these two surfaces opt into. Mobile chip-strip
`-mx-4` still cancels AuthShell's horizontal padding correctly.

src/components/admin/admin-shell.tsx

@@ -151,8 +151,13 @@ export function AdminShell({ active, children }: AdminShellProps) {
   const { t } = useTranslations();
   const activeSlug = deriveActiveSlug(pathname, active);
 
+  // v1.4.25 W8 — AuthShell wraps the page in `px-4 py-6 md:px-6`
+  // already, so this inner shell only carries the wider max-width.
+  // Previously the duplicate `px-4 py-6 md:px-6 md:py-8` here was
+  // producing visibly more top/bottom whitespace on Settings/Admin
+  // pages than on Dashboard/Insights/Measurements.
   return (
-    <div className="mx-auto w-full max-w-screen-xl px-4 py-6 md:px-6 md:py-8">
+    <div className="mx-auto w-full max-w-screen-xl">
       {/* Mobile section strip — horizontal scroll, hidden on md+.
           `no-scrollbar` (defined in `globals.css`) suppresses the
           painted scrollbar at the top of every admin page; the

src/components/settings/settings-shell.tsx

@@ -135,8 +135,13 @@ export function SettingsShell({ active, children }: SettingsShellProps) {
   const { t } = useTranslations();
   const activeSlug = deriveActiveSlug(pathname, active);
 
+  // v1.4.25 W8 — AuthShell wraps the page in `px-4 py-6 md:px-6`
+  // already, so this inner shell only carries the wider max-width.
+  // Previously the duplicate `px-4 py-6 md:px-6 md:py-8` here was
+  // producing visibly more top/bottom whitespace on Settings/Admin
+  // pages than on Dashboard/Insights/Measurements.
   return (
-    <div className="mx-auto w-full max-w-screen-xl px-4 py-6 md:px-6 md:py-8">
+    <div className="mx-auto w-full max-w-screen-xl">
       {/* Mobile section strip — horizontal scroll, hidden on md+.
           `no-scrollbar` (defined in `globals.css`) suppresses the
           painted scrollbar; the horizontal swipe + keyboard arrow