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/views/TimelineView.vue

@@ -2,6 +2,7 @@
 import { computed, onMounted, ref, watch } from "vue";
 import { api } from "../api/client";
 import { type Note } from "../stores/notes";
+import { addLocalDays, parseLocalDate } from "../notes/datetime";
 import { useNoteList } from "../composables/useNoteList";
 import { useNoteEditor } from "../composables/useNoteEditor";
 import AsyncState from "../components/AsyncState.vue";
@@ -18,23 +19,13 @@ const fromDate = ref("");
 const toDate = ref("");
 const hasRange = computed(() => !!fromDate.value || !!toDate.value);
 
-// Parse an <input type="date"> value ("YYYY-MM-DD") as a LOCAL calendar date.
-function localDate(value: string): Date | null {
-  const m = /^(\d{4})-(\d{2})-(\d{2})$/.exec(value);
-  if (!m) return null;
-  return new Date(Number(m[1]), Number(m[2]) - 1, Number(m[3]));
-}
-
 function buildQuery(): string {
   const params = new URLSearchParams({ filter: "active", sort: "created" });
-  const from = localDate(fromDate.value);
+  const from = parseLocalDate(fromDate.value);
   if (from) params.set("created_after", from.toISOString());
-  const to = localDate(toDate.value);
-  if (to) {
-    // Half-open upper bound: start of the day AFTER `to`, so the whole `to` day is included.
-    const end = new Date(to.getFullYear(), to.getMonth(), to.getDate() + 1);
-    params.set("created_before", end.toISOString());
-  }
+  const to = parseLocalDate(toDate.value);
+  // Half-open upper bound: start of the day AFTER `to`, so the whole `to` day is included.
+  if (to) params.set("created_before", addLocalDays(to, 1).toISOString());
   return params.toString();
 }