fix: ensure only one travel day per calendar date

Serialize Live-log day creation, prune empty duplicates, and use local dates for "today".

Co-authored-by: Cursor <cursoragent@cursor.com>

client/src/utils/logEntryPayload.test.ts

@@ -3,6 +3,7 @@ import {
   buildLogEntryPayload,
   hasUnsavedEventDraft,
   isLogEventDraftEmpty,
+  localDateString,
   normalizeLogEvent,
   type LogEventPayload
 } from './logEntryPayload.js'
@@ -13,6 +14,14 @@ const emptyDraft = (): LogEventPayload =>
 const filledDraft = (): LogEventPayload =>
   normalizeLogEvent({ time: '12:34', remarks: 'Wind dreht' })
 
+describe('localDateString', () => {
+  it('uses local calendar date, not UTC', () => {
+    const date = new Date(2026, 5, 4, 1, 30, 0)
+    expect(localDateString(date)).toBe('2026-06-04')
+    expect(date.toISOString().substring(0, 10)).toBe('2026-06-03')
+  })
+})
+
 describe('logEntryPayload event drafts', () => {
   it('treats time-only draft as empty', () => {
     expect(isLogEventDraftEmpty(emptyDraft())).toBe(true)

client/src/utils/logEntryPayload.ts

@@ -24,6 +24,14 @@ export interface LogEventPayload {
   remarks: string
 }
 
+/** Calendar date YYYY-MM-DD in local timezone (matches logbook entry `date` field). */
+export function localDateString(date: Date = new Date()): string {
+  const y = date.getFullYear()
+  const m = String(date.getMonth() + 1).padStart(2, '0')
+  const d = String(date.getDate()).padStart(2, '0')
+  return `${y}-${m}-${d}`
+}
+
 /** Local time as HH:MM (24-hour). */
 export function currentLocalTimeHHMM(date: Date = new Date()): string {
   const hours = String(date.getHours()).padStart(2, '0')