feat(feed,channels): "No tag" filter for untagged channels (E8·S1)

Filter to channels you haven't personally tagged in the channel manager
(no user-owned ChannelTag; auto language/topic tags don't count):

- Feed: a "No tag" sentinel in the "Your tags" sidebar combobox (exclusive
  with real tag picks) + a removable "Untagged" chip; new `untagged` bool
  feed param / `notag` URL key, persisted + coerced on every filter path.
- Channel manager: a virtual "No tag" chip in the tag-chip row filters the
  table to untagged channels (client-side, shared NO_TAG_SENTINEL).

Backend `untagged` is a plain channel-level narrowing, so feed_summary keeps
it applied in the base pass — the count/facet lockstep holds with no mirror.

Note: item E (per-tag counts) and item D (Any/All toggle) were already
shipped; S1 delivers only item B.

frontend/src/components/SavedViewsProvider.tsx

@@ -14,6 +14,7 @@ import {
   api,
   coercePlexFilters,
   coerceTagModes,
+  coerceUntagged,
   getActiveAccount,
   plexFiltersSignature,
   type FeedFilters,
@@ -42,6 +43,7 @@ export const asFilters = (raw: FeedFilters): FeedFilters => ({
   tags: raw.tags ?? [],
   channelIds: raw.channelIds ?? [],
   tagModes: coerceTagModes(raw),
+  untagged: coerceUntagged(raw),
 });
 
 type ViewSource = "feed" | "plex";