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/channelColumns.tsx

@@ -2,7 +2,7 @@ import type { ComponentProps } from "react";
 import type { TFunction } from "i18next";
 import { useTranslation } from "react-i18next";
 import { Check, Eye, EyeOff, History, UserMinus } from "lucide-react";
-import type { ManagedChannel } from "../lib/api";
+import { NO_TAG_SENTINEL, type ManagedChannel } from "../lib/api";
 import { formatCountOrDash, formatTotalHours, relativeTime } from "../lib/format";
 import type { Column } from "./DataTable";
 import Tooltip from "./Tooltip";
@@ -42,6 +42,14 @@ export function applySyncFilter(rows: ManagedChannel[], value: unknown): Managed
   return rows;
 }
 
+/** The Tags column filter predicate (item B): the "No tag" sentinel keeps channels the user hasn't
+ *  personally tagged (empty `tag_ids`); it's exclusive with real tag picks (the chip row enforces that),
+ *  so when present it wins. Otherwise a channel passes if it carries ANY selected tag id. */
+export function tagsColumnTest(c: ManagedChannel, values: string[]): boolean {
+  if (values.includes(NO_TAG_SENTINEL)) return c.tag_ids.length === 0;
+  return values.some((v) => c.tag_ids.includes(Number(v)));
+}
+
 // The Stored count, with a "fully synced" ✓ charm to the left of the number when recent uploads and
 // full history are both in — folding the retired Sync column's at-a-glance status into the value it
 // describes. Full detail stays in the hover tooltip.
@@ -289,7 +297,7 @@ export function buildChannelColumns(deps: ChannelColumnDeps): Column<ManagedChan
       filter: {
         kind: "multi",
         options: userTags.map((tg) => ({ value: String(tg.id), label: tg.name })),
-        test: (c, values) => values.some((v) => c.tag_ids.includes(Number(v))),
+        test: tagsColumnTest,
       },
       render: (c) => (
         <ChannelTagsControl