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/urlState.ts

@@ -20,6 +20,7 @@ const KEYS = [
   "tagmode_topic",
   "tagmode_other",
   "tagmode", // legacy global (topic-only) — still recognised so old shared links resolve
+  "notag",
   "channel",
   "maxage",
   "from",
@@ -44,6 +45,7 @@ export function filtersToParams(f: FeedFilters): URLSearchParams {
   for (const cat of TAG_CATEGORIES) {
     if (f.tagModes[cat] === "and") p.set(`tagmode_${cat}`, "and");
   }
+  if (f.untagged) p.set("notag", "1");
   if (f.channelIds.length) p.set("channel", f.channelIds.join(","));
   if (f.maxAgeDays) p.set("maxage", String(f.maxAgeDays));
   if (f.dateFrom) p.set("from", f.dateFrom);
@@ -95,6 +97,7 @@ export function paramsToFilters(params: URLSearchParams, base: FeedFilters): Fee
     topic: params.get("tagmode_topic") === "and" || params.get("tagmode") === "and" ? "and" : "or",
     other: params.get("tagmode_other") === "and" ? "and" : "or",
   };
+  if (params.has("notag")) f.untagged = params.get("notag") === "1";
   f.channelIds = (params.get("channel") ?? "").split(",").filter(Boolean);
   f.maxAgeDays = num(params.get("maxage"));
   f.dateFrom = params.get("from") || undefined;