fix(channels,datatable): address R13 S2a review + unify tag chips with the Tags column filter

Review fixes (user /code-review high):
- DataTable: hoist a stable EMPTY_FILTERS fallback so a controlled caller that omits
  `filters` doesn't churn a fresh {} each render.
- DataTable: the persistKey blob now stores only the slices this table OWNS — a
  controlled sort/filter is written as null/{} instead of a stale never-read duplicate.
- useFilterState: optional `migrate` seed (used only when storage is empty);
  ChannelsProvider carries the retired LS.channelFilter sync chip forward via migrateSyncChip.
- Channels: guard the sync-chip read with typeof instead of an `as string` cast.

Also unify the tag filter, same as the sync one: the top "YOUR TAGS" chip row is now a
controlled view over the Tags COLUMN filter (`filters.tags`), replacing its own tagFilter
state — so the chips and the Tags funnel share one state bidirectionally, the card layout
applies it via filterRows, and it persists with the rest of the map.

frontend/src/components/ChannelsProvider.tsx

@@ -33,6 +33,16 @@ function clampView(v: string | null): ChannelsView {
   return v === "discovery" || v === "blocked" || v === "hidden" ? v : "subscribed";
 }
 
+// One-time carry-forward of the retired single-value sync chip (LS.legacyChannelFilter, vocab
+// all/needs_full/fully_synced) into the new filter map's `sync` key (vocab ""/needsFull/fully), so a
+// user who had a chip pinned keeps it. Only consulted when channelsFilterMap is still empty.
+function migrateSyncChip(): FilterMap | null {
+  const old = getAccountRaw(LS.legacyChannelFilter);
+  if (old === "needs_full") return { sync: "needsFull" };
+  if (old === "fully_synced") return { sync: "fully" };
+  return null;
+}
+
 type ChannelsState = {
   search: string;
   // The subscribed table's column filters, lifted out of DataTable so the top status chips, the Sync
@@ -105,7 +115,7 @@ export function ChannelsProvider({ children }: { children: ReactNode }) {
   const [search, setSearchState] = useState("");
   // The subscribed table's column-filter map, owned here (lifted out of DataTable) so the top status
   // chips, the Sync column funnel, and the card layout share one state; useFilterState persists it.
-  const [filters, setFilters] = useFilterState(LS.channelsFilterMap);
+  const [filters, setFilters] = useFilterState(LS.channelsFilterMap, migrateSyncChip);
   const [view, setViewState] = useState<ChannelsView>(() =>
     clampView(getAccountRaw(LS.channelsView)),
   );