fix(channels): stable tag column width, no popover flash, no sync wrap (R8 S1 UAT)

Three UAT refinements on the tag cell + popover platform:

- Fixed tag-column width (w-[15rem], was max-w): under table-layout:auto a content-
  driven cell re-solved every column's width on each chip added, so the table visibly
  shifted (and the "+N" count flickered) until the cap was hit. A fixed footprint makes
  adding/removing tags move nothing (verified: column x unchanged, "+N" count stable).
- No popover flash: gate the floating surface's visibility on Floating UI's isPositioned
  so it never paints at (0,0) top-left for a frame before the transform lands.
- Sync cell no longer wraps: flex-nowrap so the recent/full badges stay on one line
  (the column grows to fit them instead of breaking to two rows).

frontend/src/components/Channels.tsx

@@ -1003,7 +1003,7 @@ function SyncCell({ c }: { c: ManagedChannel }) {
     );
   }
   return (
-    <div className="flex flex-wrap items-center gap-1">
+    <div className="flex flex-nowrap items-center gap-1 whitespace-nowrap">
       <SyncBadge
         ok={c.recent_synced}
         label={t("channels.row.recent")}