refactor: split admin.css monolith, standardize CSS architecture

- Split admin.css (3228 lines) into 12 focused files: variables, base,
  forms, buttons, layout, components, tables, skeleton, datepicker,
  filemanager, pagination, responsive
- Extracted shared styles from offers.css and dashboard.css into
  components.css and forms.css (offers-* → admin-* prefix)
- Standardized naming: dash-kpi-* → admin-kpi-*, session-* → dash-session-*,
  rich-editor → admin-rich-editor
- Deleted duplicate offers-tabs (using admin-tabs everywhere)
- Deduplicated DatePicker and FileManager CSS (~360 lines removed)
- Added 16 utility classes to base.css (font sizes, widths, gaps, margins)
- Deleted empty admin.css

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/admin/pages/LeaveApproval.tsx

@@ -313,9 +313,9 @@ export default function LeaveApproval() {
         animate={{ opacity: 1, y: 0 }}
         transition={{ duration: 0.25, delay: 0.06 }}
       >
-        <div className="offers-tabs mb-6">
+        <div className="admin-tabs mb-6">
           <button
-            className={`offers-tab ${activeTab === "pending" ? "active" : ""}`}
+            className={`admin-tab ${activeTab === "pending" ? "active" : ""}`}
             onClick={() => setActiveTab("pending")}
           >
             Ke schválení
@@ -333,7 +333,7 @@ export default function LeaveApproval() {
             )}
           </button>
           <button
-            className={`offers-tab ${activeTab === "processed" ? "active" : ""}`}
+            className={`admin-tab ${activeTab === "processed" ? "active" : ""}`}
             onClick={() => setActiveTab("processed")}
           >
             Vyřízené