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/lib/api/feed.ts

@@ -71,6 +71,9 @@ export interface FeedFilters {
   // Library (scope "all") only — provenance filter: "organic" (default) hides search-
   // discovered videos, "all" mixes them in, "search" shows only them. Ignored for "my".
   librarySource?: "organic" | "all" | "search" | "plex";
+  /** "No tag" filter (item B): keep only channels the user has NOT personally tagged in the channel
+   *  manager (no user-owned ChannelTag). Auto/system tags don't count. */
+  untagged?: boolean;
   /** OR-ed together: a video from ANY of these passes, and every other filter still applies. */
   channelIds: string[];
   maxAgeDays?: number;
@@ -89,6 +92,7 @@ function filterParams(f: FeedFilters): URLSearchParams {
   for (const cat of TAG_CATEGORIES) {
     if (f.tagModes[cat] === "and") p.set(`tag_mode_${cat}`, "and");
   }
+  if (f.untagged) p.set("untagged", "true");
   if (f.q) p.set("q", f.q);
   p.set("scope", f.scope);
   p.set("show_normal", String(f.includeNormal));