M9 S3 (frontend): consolidate local-date helpers into notes/datetime.ts

The created-date range facets built local-day bounds by hand in two places
(FilterBar's onTo/toInput, TimelineView's buildQuery) — parse a
"YYYY-MM-DD", shift a day for the half-open upper bound, format back.

- datetime.ts: parseLocalDate() / addLocalDays() (non-mutating) / formatLocalDay().
- TimelineView: drops its inline localDate() + the +1-day Date math.
- FilterBar: drops its inline isoDay() + the setDate(±1) mutations.

Behavior-preserving and deliberately NOT unifying output: Timeline still
emits UTC (.toISOString()) bounds, FilterBar still emits naive-local
"…T00:00:00" strings — only the shared primitives are extracted. (The
naive-vs-UTC divergence is a separate backend-datetime-semantics question,
flagged for later, not silently changed.)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

frontend/src/components/FilterBar.vue

@@ -6,6 +6,7 @@ import { useSavedFiltersStore } from "../stores/savedFilters";
 import { useUiStore } from "../stores/ui";
 import type { NoteFacets } from "../stores/notes";
 import { facetCount, facetsFromQuery, facetsToQuery } from "../notes/facets";
+import { addLocalDays, formatLocalDay, parseLocalDate } from "../notes/datetime";
 import { NOTE_COLOR_KEYS, NOTE_COLOR_LABELS, NOTE_SWATCH_CLASSES, type NoteColor } from "../notes/colors";
 import Icon from "./Icon.vue";
 
@@ -56,30 +57,25 @@ function onQ(e: Event) {
   qTimer = setTimeout(() => patch({ q: v.trim() || undefined }), 300);
 }
 
-function isoDay(d: Date): string {
-  return `${d.getFullYear()}-${String(d.getMonth() + 1).padStart(2, "0")}-${String(d.getDate()).padStart(2, "0")}`;
-}
 function onFrom(e: Event) {
   const v = (e.target as HTMLInputElement).value;
   patch({ created_after: v ? `${v}T00:00:00` : undefined });
 }
 function onTo(e: Event) {
   const v = (e.target as HTMLInputElement).value;
-  if (!v) {
+  const d = v ? parseLocalDate(v) : null;
+  if (!d) {
     patch({ created_before: undefined });
     return;
   }
   // Half-open upper bound: the start of the day AFTER the chosen date (so it's inclusive).
-  const d = new Date(`${v}T00:00:00`);
-  d.setDate(d.getDate() + 1);
-  patch({ created_before: `${isoDay(d)}T00:00:00` });
+  patch({ created_before: `${formatLocalDay(addLocalDays(d, 1))}T00:00:00` });
 }
 const fromInput = computed(() => (facets.value.created_after ?? "").slice(0, 10));
 const toInput = computed(() => {
-  if (!facets.value.created_before) return "";
-  const d = new Date(facets.value.created_before);
-  d.setDate(d.getDate() - 1);
-  return isoDay(d);
+  // Reverse the half-open bound (day-after) back to the chosen end day for the input.
+  const d = facets.value.created_before ? parseLocalDate(facets.value.created_before.slice(0, 10)) : null;
+  return d ? formatLocalDay(addLocalDays(d, -1)) : "";
 });
 
 async function saveView() {