refactor(ui): fold the feed TagChip onto the shared filterChipClass (R13 S2c)

The feed sidebar's multi-select TagChip (language/topic/tag chips) was the last
hand-rolled chip whose className duplicated the canonical geometry verbatim
(it's the reference look filterChipClass was derived from). Point it at the
shared constant; the only delta is dropping a one-off hover:shadow-sm so every
filter chip in the app now hovers identically. Badge markup unchanged.
gate green (tsc/eslint-0/prettier). Branch not merged.

frontend/src/components/Sidebar.tsx

@@ -16,7 +16,7 @@ import SidePanel from "./SidePanel";
 import PanelGroups, { type PanelItem } from "./PanelGroups";
 import ChannelPicker from "./ChannelPicker";
 import SegmentedToggle from "./SegmentedToggle";
-import FilterChipGroup from "./FilterChipGroup";
+import FilterChipGroup, { filterChipClass } from "./FilterChipGroup";
 import { useFeedFilters, useFeedFiltersActions } from "./FeedFiltersProvider";
 import { useChannelsActions } from "./ChannelsProvider";
 import { useMe } from "../lib/useMe";
@@ -49,7 +49,7 @@ function TagChip({
       onClick={onClick}
       title={t("sidebar.channelCount", { count })}
       aria-pressed={active}
-      className="pill inline-flex items-center gap-1.5 text-xs px-2.5 py-1 rounded-full border shadow-xs hover:shadow-sm active:translate-y-px"
+      className={filterChipClass}
     >
       <span>{tag.name}</span>
       <span