Make Follows/Global and All/Mentions tabs sticky on mobile (top-12)

Tab bars should stick below the mobile top bar (h-12 = top-12) on mobile
and at top-0 on desktop. These are navigation tabs, not page title headers.

Co-authored-by: shakespeare.diy <assistant@shakespeare.diy>

src/components/Feed.tsx

@@ -101,7 +101,7 @@ export function Feed() {
 
       {/* Tabs (logged in) or CTA (logged out) */}
       {user ? (
-        <div className="flex border-b border-border sidebar:sticky sidebar:top-0 bg-background/80 backdrop-blur-md z-10">
+        <div className="flex border-b border-border sticky top-12 sidebar:top-0 bg-background/80 backdrop-blur-md z-10">
           <TabButton
             label="Follows"
             active={activeTab === 'follows'}
@@ -114,7 +114,7 @@ export function Feed() {
           />
         </div>
       ) : (
-        <div className="border-b border-border sidebar:sticky sidebar:top-0 bg-gradient-to-r from-primary/5 via-primary/10 to-primary/5 backdrop-blur-md z-10 py-3">
+        <div className="border-b border-border sticky top-12 sidebar:top-0 bg-gradient-to-r from-primary/5 via-primary/10 to-primary/5 backdrop-blur-md z-10 py-3">
           <div className="flex items-center justify-center gap-3 px-6">
             <p className="text-[13px] sidebar:text-sm text-muted-foreground">
               Follow accounts you care about on Mew

src/pages/NotificationsPage.tsx

@@ -75,7 +75,7 @@ export function NotificationsPage() {
         </div>
 
         {/* Tab bar */}
-        <div className="flex border-b border-border sidebar:sticky sidebar:top-0 bg-background/80 backdrop-blur-md z-10">
+        <div className="flex border-b border-border sticky top-12 sidebar:top-0 bg-background/80 backdrop-blur-md z-10">
           {tabs.map(({ key, label }) => (
             <button
               key={key}