Finish the redesign's edges: auth page, modals, dead code, docs

The sign-in page becomes a plain centred column with the brand mark
and a segmented login/register switch, modals lose their last inner
boxes, the red-days list collapses every year but the current one,
and the retired components' CSS, an unused list component and 23
unreferenced translation keys are removed. ARCHITECTURE.md documents
the Almanacka styling conventions and the per-year balance rule.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XBRLomdLK9jF7wU7a8AA6H

frontend/src/i18n/en/calendar.ts

@@ -1,15 +1,8 @@
 export const calendar = {
-  'calendar.advanceMonthAria': '{month} - click to advance month',
-  'calendar.advanceYearAria': '{year} - click to advance year',
   'calendar.selectMonthAria': 'Select month',
   'calendar.selectYearAria': 'Select year',
   'calendar.deleteError': 'Failed to delete the planned day',
-  'calendar.loading': 'Loading calendar...',
   'calendar.nextMonthAria': 'Next month',
-  'calendar.overrideError': 'Failed to update the exception',
-  'calendar.overrideModeActiveLabel': 'Stop editing days',
-  'calendar.overrideModeHint': 'Click a day to mark it as a working day or a day off. Click a marked day again to reset it.',
-  'calendar.overrideModeLabel': 'Mark working/day off',
   'calendar.partner': 'Partner',
   'calendar.prevMonthAria': 'Previous month',
   'calendar.saveError': 'Failed to save the planned days',
@@ -46,7 +39,6 @@ export const calendar = {
   'plannedDayModal.pinnedNotice': 'This day is pinned by you — the planner will not change it automatically.',
   'plannedDayModal.excludedNotice': 'You removed this day for {parent}.',
   'plannedDayModal.restoreExcludedButton': 'Put the day back',
-  'plannedDayModal.dayStatusLabel': 'This day is:',
   'plannedDayModal.rangeTitle': 'Plan {count} days',
   'plannedDayModal.parentModeLabel': 'Who is this for?',
   'plannedDayModal.wantsDubbeldagLabel': 'Allow double days (dubbeldagar)',
@@ -75,7 +67,6 @@ export const calendar = {
 
   // §Task7 — the calendar becomes the plan: drag-select, period popover,
   // preset rail, and the sync toast/undo.
-  'calendar.newPeriodName': 'New period',
   'calendar.periodCreateError': 'Failed to create the period',
   'calendar.syncError': 'Failed to sync the plan',
   'calendar.syncingLabel': 'Syncing…',
@@ -118,7 +109,6 @@ export const calendar = {
   'calendar.presetChipsLabel': 'Add leave',
   'calendar.presetWeeksShort': '{count} wk',
 
-  'calendar.periodPopoverTitle': 'Edit period',
   'calendar.periodAdHocTitle': 'Period',
   'calendar.periodAdHocNotice': 'This period is not linked to a saved wish.',
   'calendar.periodNameLabel': 'Name',
@@ -141,9 +131,7 @@ export const calendar = {
   'calendar.periodLastYearNeedsSeriesHint': 'Choose "Every year" to change the last year.',
   'calendar.periodSaveButton': 'Save period',
   'calendar.periodSavingButton': 'Saving…',
-  'calendar.periodDeleteButton': 'Delete period',
   'calendar.periodDeletingButton': 'Deleting…',
-  'calendar.periodDeleteConfirm': 'Are you sure you want to delete this period?',
   'calendar.periodDeleteConfirmButton': 'Yes, delete',
   'calendar.periodDeleteAdHocButton': 'Delete these days',
   'calendar.periodCancelButton': 'Cancel',