Prevent overlaps in quick-entry and validate max capacity in a day - #6192

Description

Fixes #5348: Quick entry gave every row of a day the same configured default start
time and saved the resulting overlaps, even though overlapping records are forbidden.

Root cause: QuickEntryController stamped every new grid row for every day with the
same default begin time, and the existing TimesheetOverlapping per-row constraint
only queries already-persisted records — it cannot see the other unsaved rows in the
same grid submission, so an overlap created inside one submission was invisible.

Fix: a new App\Validator\Constraints\QuickEntryWeek class-level constraint (with its
QuickEntryWeekValidator), wired into QuickEntryForm, that for each user/day group
in a quick-entry submission:

• chains new rows back to back, starting after whatever the day already holds
  (TimesheetRepository::findForDay(), including a still-running timer that began an
  earlier day);
• rejects the day with the same "You already have an entry for this time." message the
  single-entry form uses when a running timer would otherwise be silently overlapped;
• rejects a day whose total duration (existing + submitted) exceeds that calendar day's
  actual DST-aware length, or whose chained end would cross midnight, naming the day
  (new translations/validators.en.xlf entry).

Existing (already-persisted) rows are never repositioned or checked against each other,
so a pre-existing overlapping pair remains editable without one side blocking the other.
allow_overlapping_records = 1 installations see no behavioural change beyond the new
day-capacity limit. TimesheetService::updateMultipleTimesheets() and the
single-entry/API paths are untouched.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I verified that my code applies to the guidelines (composer code-check: codestyle,
  phpstan level 9, linting all exit 0)
• I updated the documentation (see here)
• I agree that this code is used in Kimai (see license)

How this was verified

• New tests: tests/Validator/Constraints/QuickEntryWeekValidatorTest.php (19 cases —
  chaining, day-sum boundaries, DST spring-forward/fall-back, running-timer block
  (same-day and begun-the-day-before), allow_overlapping_records=1 bypass, cleared
  duration, existing-overlapping-pair tolerance, multi-user isolation, translation
  domain wiring), tests/Repository/TimesheetRepositoryTest.php (new findForDay()
  coverage), tests/Controller/QuickEntryControllerTest.php (end-to-end reproduction +
  billing check via real HTTP submission).
• All 18 of the task's acceptance criteria are checked off with evidence in
  tasks/BUG-5348-quick-entry-same-start-time-overlap.md.
• Two rounds of an independent code-review pass found and fixed: a findForDay() gap
  for a still-running timer that began the previous day, a phpstan nullable-call gate
  failure in test code, and a missing translation domain on the day-capacity violation
  message.
• Regression run: tests/Timesheet/TimesheetServiceTest.php,
  tests/Validator/Constraints/TimesheetOverlappingValidatorTest.php,
  tests/Model/QuickEntryWeekTest.php, tests/Model/QuickEntryModelTest.php,
  tests/Form/Type/QuickEntryTimesheetTypeTest.php,
  tests/Validator/Constraints/QuickEntryModelValidatorTest.php,
  tests/Validator/Constraints/QuickEntryTimesheetValidatorTest.php — all green.