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/components/PlanCategoriesModal.tsx

@@ -1,4 +1,4 @@
-import { useState } from "react";
+import { useState, type ChangeEvent, type FocusEvent } from "react";
 import Box from "@mui/material/Box";
 import { Modal, TextField, Button } from "../ui";
 import { useApiMutation } from "../lib/queries/mutations";
@@ -100,6 +100,19 @@ export default function PlanCategoriesModal({
     py: 1,
   } as const;
 
+  // Native color picker, styled to match the row (was the .plan-cat-color rule).
+  const colorInputSx = {
+    width: 34,
+    height: 34,
+    p: 0,
+    border: 1,
+    borderColor: "divider",
+    borderRadius: "6px",
+    background: "none",
+    cursor: "pointer",
+    flex: "0 0 auto",
+  } as const;
+
   return (
     <Modal
       isOpen={isOpen}
@@ -113,9 +126,10 @@ export default function PlanCategoriesModal({
       <Box>
         {categories.map((c) => (
           <Box key={c.id} sx={{ ...rowSx, opacity: c.is_active ? 1 : 0.55 }}>
-            <input
+            <Box
+              component="input"
               type="color"
-              className="plan-cat-color"
+              sx={colorInputSx}
               // Uncontrolled + patch on blur (like the rename input): the
               // native color picker fires onChange on every drag tick, which
               // would spam PATCH requests and the audit log. onBlur commits
@@ -123,7 +137,7 @@ export default function PlanCategoriesModal({
               defaultValue={c.color}
               key={c.color}
               disabled={busy}
-              onBlur={(e) => {
+              onBlur={(e: FocusEvent<HTMLInputElement>) => {
                 if (e.target.value !== c.color) {
                   patch(c.id, { color: e.target.value });
                 }
@@ -190,12 +204,15 @@ export default function PlanCategoriesModal({
         <Box
           sx={{ ...rowSx, mt: 1, pt: 2, borderTop: 1, borderColor: "divider" }}
         >
-          <input
+          <Box
+            component="input"
             type="color"
-            className="plan-cat-color"
+            sx={colorInputSx}
             value={newColor}
             disabled={busy}
-            onChange={(e) => setNewColor(e.target.value)}
+            onChange={(e: ChangeEvent<HTMLInputElement>) =>
+              setNewColor(e.target.value)
+            }
             aria-label="Barva nové kategorie"
           />
           <TextField