refactor(css): delete dead pre-MUI CSS and eliminate plan.css

Phase 1 of the "fully MUI" CSS cleanup.

- base.css 429->317: drop dead .admin-sidebar/header/card/modal,
  .admin-drag-handle/-kbd/-mono/-error-stack/-loading and the unused
  float/pulse keyframes. Keep reset, typography, scrollbar,
  ::view-transition cross-fade, the live .admin-spinner (App.tsx
  pre-React loader) and all utility classes.
- attendance.css 456->24: keep only the Leaflet .attendance-location-map
  rules; the old hand-rolled Attendance page is MUI now.
- offers.css 642->417: keep only the Quill (.admin-rich-editor/.ql-*)
  and .admin-rich-text-view rules; drop the dead offers table / filter /
  lang-badge / row-state rules.
- plan.css deleted: the grid already lives in PlanGrid's styled()
  wrapper; the last consumer (.plan-cat-color) is now an sx-styled
  Box input in PlanCategoriesModal.

tsc -b --noEmit and npm run build both clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/admin/base.css

@@ -53,16 +53,6 @@ body {
     color 0.3s ease;
 }
 
-.admin-sidebar,
-.admin-header,
-.admin-card,
-.admin-modal {
-  transition:
-    background-color 0.3s ease,
-    color 0.3s ease,
-    border-color 0.3s ease;
-}
-
 #root {
   overflow-x: hidden;
   touch-action: pan-y pinch-zoom;
@@ -230,80 +220,6 @@ img {
   text-align: center;
 }
 
-/* Spinner variant */
-.admin-spinner-sm {
-  width: 16px;
-  height: 16px;
-  border-width: 2px;
-}
-
-/* Monospace for data values (times, dates, numbers, IDs) */
-.admin-mono {
-  font-family: var(--font-mono);
-  font-size: 0.875em;
-  letter-spacing: -0.01em;
-}
-
-/* Drag handle */
-.admin-drag-handle {
-  display: flex;
-  align-items: center;
-  justify-content: center;
-  width: 24px;
-  height: 24px;
-  border: none;
-  background: none;
-  color: var(--text-muted);
-  cursor: grab;
-  border-radius: 4px;
-  padding: 0;
-  transition:
-    color 0.15s,
-    background 0.15s;
-  touch-action: none;
-}
-
-.admin-drag-handle:hover {
-  color: var(--text-primary);
-  background: var(--bg-secondary);
-}
-
-.admin-drag-handle:active {
-  cursor: grabbing;
-}
-
-/* Error stack (DEV only) */
-.admin-error-stack {
-  max-width: 600px;
-  max-height: 200px;
-  overflow: auto;
-  padding: 0.75rem 1rem;
-  margin: 0;
-  border-radius: var(--border-radius-sm);
-  background: var(--bg-tertiary);
-  border: 1px solid var(--border-color);
-  color: var(--danger);
-  font-family: var(--font-mono);
-  font-size: 11px;
-  line-height: 1.5;
-  text-align: left;
-  white-space: pre-wrap;
-  word-break: break-word;
-}
-
-/* Keyboard shortcut badge */
-.admin-kbd {
-  display: inline-block;
-  padding: 2px 7px;
-  font-family: var(--font-mono);
-  font-size: 12px;
-  line-height: 1.4;
-  border-radius: 4px;
-  background: var(--bg-secondary);
-  border: 1px solid var(--border-color);
-  white-space: nowrap;
-}
-
 /* Loading & Animations */
 .admin-spinner {
   width: 32px;
@@ -314,39 +230,12 @@ img {
   animation: spin 0.8s linear infinite;
 }
 
-.admin-loading {
-  display: flex;
-  align-items: center;
-  justify-content: center;
-  min-height: 256px;
-}
-
 @keyframes spin {
   to {
     transform: rotate(360deg);
   }
 }
 
-@keyframes float {
-  0%,
-  100% {
-    transform: translate(0, 0);
-  }
-  50% {
-    transform: translate(30px, -30px);
-  }
-}
-
-@keyframes pulse {
-  0%,
-  100% {
-    opacity: 1;
-  }
-  50% {
-    opacity: 0.5;
-  }
-}
-
 /* ── Additional Utilities ─────────────────────────────────────────── */
 
 /* Font sizes */