fix(feed): stop the layout juddering when content lands on the scroll threshold

The feed takes its column count from main's clientWidth and its
thumbnails are aspect-ratio boxes, so the two feed each other once the
content height sits within a scrollbar's width of the viewport: the bar
appears, 15px goes, thumbs shorten, the content fits, the bar leaves,
and round again — every frame. Measured: scrollbar width alternating
0/15 and clientWidth 987/972 on consecutive frames. Reserving the gutter
holds clientWidth constant and breaks it.

frontend/src/App.tsx

@@ -898,7 +898,13 @@ export default function App() {
         >
         {meQuery.data!.is_demo && <DemoBanner />}
         <VersionBanner onOpen={() => openReleaseNotes(CURRENT_VERSION)} />
-          <main className="flex-1 min-w-0 overflow-y-auto">
+          {/* scrollbar-gutter: stable is load-bearing, not cosmetic. The feed derives its column
+              count from this element's clientWidth and its thumbnails are aspect-ratio boxes, so
+              when the content lands within a scrollbar's width of the viewport height the two feed
+              each other: scrollbar appears -> 15px narrower -> shorter thumbs -> content fits ->
+              scrollbar goes -> wider -> taller -> repeat, every frame. Reserving the gutter keeps
+              clientWidth constant and breaks the loop. */}
+          <main className="flex-1 min-w-0 overflow-y-auto [scrollbar-gutter:stable]">
           <Suspense fallback={pageFallback}>
           {page === "channels" ? (
             <Channels