feat(trips): add configurable day count for trips without dates

- Show day count input in trip form when no start/end date is set
- Backend accepts day_count param for create and update
- Remove forced date assignment for dateless trips (was always setting tomorrow + 7)
- Fix off-by-one: single-date fallback now creates 7 days instead of 8
- Add dayCount/dayCountHint translations for all 13 languages

client/src/i18n/translations/en.ts

@@ -113,6 +113,8 @@ const en: Record<string, string | { name: string; category: string }[]> = {
   'dashboard.tripDescriptionPlaceholder': 'What is this trip about?',
   'dashboard.startDate': 'Start Date',
   'dashboard.endDate': 'End Date',
+  'dashboard.dayCount': 'Number of Days',
+  'dashboard.dayCountHint': 'How many days to plan for when no travel dates are set.',
   'dashboard.noDateHint': 'No date set — 7 default days will be created. You can change this anytime.',
   'dashboard.coverImage': 'Cover Image',
   'dashboard.addCoverImage': 'Add cover image (or drag & drop)',