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/styles/responsive.css

@@ -84,25 +84,6 @@
     max-width: none;
   }
 
-  .form-grid {
-    grid-template-columns: 1fr;
-  }
-
-  .form-grid .form-group input,
-  .form-grid .form-group select {
-    max-width: none;
-  }
-
-  /* Tables */
-  .vacation-days-table {
-    font-size: var(--fs-sm);
-  }
-
-  .vacation-days-table th,
-  .vacation-days-table td {
-    padding: var(--sp-2) var(--sp-3);
-  }
-
   /* Dashboard — the three key figures tighten; a year row keeps year +
    * track on the first line and wraps the per-parent figures under the
    * track, each parent on its own line. */
@@ -143,10 +124,6 @@
     width: 100%;
   }
 
-  .vacation-days-table__input {
-    width: 70px;
-  }
-
   /* Modals — the overlay already gutters the content with its own padding
    * and `.modal__content`'s `width: min(100%, var(--modal-w))` already caps
    * it to the viewport, so only the footer's button stacking needs a
@@ -228,10 +205,6 @@
 
 /* Tablet - Medium screens (641px to 1024px) */
 @media (min-width: 641px) and (max-width: 1024px) {
-  .form-grid {
-    grid-template-columns: repeat(2, 1fr);
-  }
-
   .layout__main {
     padding: var(--sp-6) var(--sp-5);
   }