test(sidebar): cover the date preset/custom selection logic (R13 S2c)

The deep-review flagged that the feed sidebar's new date single-select
derivation was the one bit of migrated logic without a test, while its sibling
primitives (FilterChipGroup/SegmentedToggle/SortControl) all gained one. Lift
the pure logic into lib/dateFilter (deriveDateKey + applyDateChip), same split
as columnSort, and cover it: dateKey for none/preset/custom/off-preset-value,
the preset toggle-off, the preset-not-active-while-custom-open case, and the
Custom open/close + maxAgeDays clearing. Sidebar now calls the helpers; the
onChange is behaviour-identical (self-E2E'd: preset on/off, Custom editor).
11 new tests (vitest 136 -> 147). gate green. Branch not merged.

frontend/src/components/Sidebar.tsx

@@ -10,6 +10,7 @@ import { copyText } from "../lib/clipboard";
 import { notify } from "../lib/notifications";
 import { useDebounced } from "../lib/useDebounced";
 import { clearedFilters, useActiveFilters } from "../lib/useActiveFilters";
+import { applyDateChip, deriveDateKey } from "../lib/dateFilter";
 import TagManager from "./TagManager";
 import SavedViewsWidget from "./SavedViewsWidget";
 import SidePanel from "./SidePanel";
@@ -152,17 +153,8 @@ export default function Sidebar({
       case "channel":
         return <ChannelPicker filters={filters} setFilters={setFilters} />;
       case "date": {
-        // Single-select across the presets + Custom: a preset is active only when its maxAge matches
-        // and no custom range is open; otherwise Custom owns the selection (or none).
-        const activePresetDays = DATE_PRESETS.find(
-          (p) => filters.maxAgeDays === p.days && !customDates,
-        )?.days;
-        const dateKey =
-          customDates || filters.dateFrom || filters.dateTo
-            ? "custom"
-            : activePresetDays != null
-              ? String(activePresetDays)
-              : "";
+        // Single-select across the presets + Custom; pure logic lives in lib/dateFilter.
+        const dateKey = deriveDateKey(filters, customDates, DATE_PRESETS);
         return (
           <>
             <FilterChipGroup
@@ -176,20 +168,9 @@ export default function Sidebar({
                 { value: "custom", label: t("sidebar.custom") },
               ]}
               onChange={(key) => {
-                if (key === "custom") {
-                  const next = !customDates;
-                  setCustomDates(next);
-                  if (next) setFilters({ ...filters, maxAgeDays: undefined });
-                  return;
-                }
-                const days = Number(key);
-                const activePreset = filters.maxAgeDays === days && !customDates;
-                setFilters({
-                  ...filters,
-                  maxAgeDays: activePreset ? undefined : days,
-                  dateFrom: undefined,
-                  dateTo: undefined,
-                });
+                const change = applyDateChip(key, filters, customDates);
+                if (change.customDates !== undefined) setCustomDates(change.customDates);
+                if (change.filters) setFilters(change.filters);
               }}
             />