improvement(scroll): fade every capped list's clipped edges

The sweep's own three (the thread, the Plex collections filter, the tag manager)
plus the seven the spec missed — the playlist picker, the row tag menu, the Plex
playlist/collection dialogs, the player's track menu, the video description and
the table's multi-select filter. Same idiom throughout: a height-capped list that
silently clipped its content now says so, and hides its scrollbar like the rail
and the panels already do.

The table's option list is its own module-level component because it owns a hook:
FilterPopover is redeclared per DataTable render, so React remounts it.

frontend/src/components/Channels.tsx

@@ -19,6 +19,7 @@ import { api, type ManagedChannel, type Tag } from "../lib/api";
 import { notifyYouTubeActionError } from "../lib/youtubeErrors";
 import { accountKey, LS } from "../lib/storage";
 import { useDismiss } from "../lib/useDismiss";
+import { useScrollFade } from "../lib/useScrollFade";
 import { formatEta, formatTotalHours, relativeTime } from "../lib/format";
 import { subsColumn } from "./channelColumns";
 import { notify } from "../lib/notifications";
@@ -684,6 +685,7 @@ function TagsCell({
   // Open the picker upward when there isn't room below (rows near the viewport bottom would
   // otherwise clip it against the scroll container), as long as there's room above.
   const [openUp, setOpenUp] = useState(false);
+  const fade = useScrollFade();
   const ref = useRef<HTMLDivElement | null>(null);
   const btnRef = useRef<HTMLButtonElement | null>(null);
   useDismiss(open, () => setOpen(false), ref);
@@ -723,7 +725,9 @@ function TagsCell({
       </button>
       {open && (
         <div
-          className={`glass-menu absolute left-0 z-30 w-44 max-h-[264px] overflow-y-auto rounded-xl p-1.5 animate-[popIn_0.16s_ease] ${
+          ref={fade.ref}
+          style={fade.style}
+          className={`glass-menu absolute left-0 z-30 w-44 max-h-[264px] overflow-y-auto no-scrollbar rounded-xl p-1.5 animate-[popIn_0.16s_ease] ${
             openUp ? "bottom-full mb-1" : "top-full mt-1"
           }`}
         >