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/Sidebar.tsx

@@ -3,7 +3,7 @@ import { useTranslation } from "react-i18next";
 import { keepPreviousData, useQuery } from "@tanstack/react-query";
 import { qk } from "../lib/queryKeys";
 import { SlidersHorizontal } from "lucide-react";
-import { api, type FeedFilters, type Tag, type TagCategory } from "../lib/api";
+import { api, NO_TAG_SENTINEL, type FeedFilters, type Tag, type TagCategory } from "../lib/api";
 import { defaultLayout, type PanelLayout } from "../lib/panelLayout";
 import { useDebounced } from "../lib/useDebounced";
 import { useActiveFilters } from "../lib/useActiveFilters";
@@ -104,6 +104,41 @@ export default function Sidebar({
   const setTagMode = (category: TagCategory, mode: "or" | "and") =>
     setFilters({ ...filters, tagModes: { ...filters.tagModes, [category]: mode } });
 
+  // The "Your tags" combobox carries the "No tag" sentinel (item B) at the top: selecting it filters to
+  // channels you HAVEN'T personally tagged in the channel manager (`filters.untagged`). It's mutually
+  // exclusive with picking real user tags — choosing it clears your tag picks, and picking a tag clears
+  // it. Language/Topic are auto/system tags, so "No tag" is meaningless there and only lives here.
+  function userTagsCombo(): ReactNode {
+    const noneOn = !!filters.untagged;
+    const inList = (id: number) => userTags.some((tg) => tg.id === id);
+    const onChange = (next: string[]) => {
+      if (next.includes(NO_TAG_SENTINEL) && !noneOn) {
+        setFilters({ ...filters, tags: filters.tags.filter((id) => !inList(id)), untagged: true });
+      } else {
+        const reals = next.filter((v) => v !== NO_TAG_SENTINEL).map(Number);
+        setFilters({
+          ...filters,
+          tags: [...filters.tags.filter((id) => !inList(id)), ...reals],
+          untagged: false,
+        });
+      }
+    };
+    return (
+      <Combobox
+        label={t("sidebar.widget.tags")}
+        options={[
+          { value: NO_TAG_SENTINEL, label: t("sidebar.noTagOption") },
+          ...visibleChips(userTags).map(tagOption),
+        ]}
+        selected={noneOn ? [NO_TAG_SENTINEL] : selectedIn(userTags)}
+        onChange={onChange}
+        emptyText={t("sidebar.noMatchingTags")}
+        matchMode={filters.tagModes.other}
+        onMatchModeChange={(m) => setTagMode("other", m)}
+      />
+    );
+  }
+
   // The badge counts exactly what the chip row above the feed shows — same list, so they can't
   // disagree about what's applied.
   const activeCount = useActiveFilters(filters, setFilters).length;
@@ -124,7 +159,7 @@ export default function Sidebar({
         return (
           <DateFilterSelect filters={filters} setFilters={setFilters} presets={DATE_PRESETS} />
         );
-      case "language": {
+      case "language":
         return (
           <Combobox
             label={t("sidebar.widget.language")}
@@ -136,8 +171,7 @@ export default function Sidebar({
             onMatchModeChange={(m) => setTagMode("language", m)}
           />
         );
-      }
-      case "topic": {
+      case "topic":
         return (
           <Combobox
             label={t("sidebar.widget.topic")}
@@ -149,22 +183,10 @@ export default function Sidebar({
             onMatchModeChange={(m) => setTagMode("topic", m)}
           />
         );
-      }
-      case "tags": {
+      case "tags":
         // The widget's own "Manage" trigger is its title (see the PanelItem onTitleClick below), so the
-        // body is just the picker.
-        return (
-          <Combobox
-            label={t("sidebar.widget.tags")}
-            options={visibleChips(userTags).map(tagOption)}
-            selected={selectedIn(userTags)}
-            onChange={(next) => setTagSelection(userTags, next)}
-            emptyText={t("sidebar.noMatchingTags")}
-            matchMode={filters.tagModes.other}
-            onMatchModeChange={(m) => setTagMode("other", m)}
-          />
-        );
-      }
+        // body is just the picker — user tags plus the "No tag" sentinel.
+        return userTagsCombo();
       default:
         return null;
     }