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/lib/storage.ts

@@ -26,6 +26,9 @@ export const LS = {
   // chips, the Sync column funnel, and the card layout all share ONE state (per-account). Supersedes
   // the old single-string `channelFilter` sync chip.
   channelsFilterMap: "siftlode.channelsFilterMap",
+  // DEPRECATED — the retired single-value sync chip; read once to migrate a pinned value into
+  // `channelsFilterMap`, then inert. Do not write.
+  legacyChannelFilter: "siftlode.channelFilter",
   channelsView: "siftlode.channelsView",
   // The channel manager's layout (table / cards — see lib/channelLayout.ts), per-account.
   channelLayout: "siftlode.channelLayout",