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/Invoices.tsx

@@ -334,7 +334,7 @@ export default function Invoices() {
               style={{ width: "140px", borderRadius: "8px" }}
             />
           </div>
-          <div className="dash-kpi-grid dash-kpi-4">
+          <div className="admin-kpi-grid admin-kpi-4">
             {[0, 1, 2, 3].map((i) => (
               <div key={i} className="admin-stat-card">
                 <div
@@ -497,15 +497,15 @@ export default function Invoices() {
           </button>
         </div>
 
-        <div className="offers-tabs mb-4" style={{ justifyContent: "center" }}>
+        <div className="admin-tabs mb-4" style={{ justifyContent: "center" }}>
           <button
-            className={`offers-tab ${activeTab === "issued" ? "active" : ""}`}
+            className={`admin-tab ${activeTab === "issued" ? "active" : ""}`}
             onClick={() => setActiveTab("issued")}
           >
             Vydané
           </button>
           <button
-            className={`offers-tab ${activeTab === "received" ? "active" : ""}`}
+            className={`admin-tab ${activeTab === "received" ? "active" : ""}`}
             onClick={() => setActiveTab("received")}
           >
             Přijaté
@@ -522,7 +522,7 @@ export default function Invoices() {
           <Suspense
             fallback={
               <div
-                className="dash-kpi-grid dash-kpi-4"
+                className="admin-kpi-grid admin-kpi-4"
                 style={{ marginBottom: "1.5rem" }}
               >
                 {[0, 1, 2, 3].map((i) => (
@@ -569,7 +569,7 @@ export default function Invoices() {
           >
             {!hasLoadedOnce.current && statsLoading ? (
               <div
-                className="dash-kpi-grid dash-kpi-4"
+                className="admin-kpi-grid admin-kpi-4"
                 style={{ marginBottom: "1.5rem" }}
               >
                 {[0, 1, 2, 3].map((i) => (
@@ -607,7 +607,7 @@ export default function Invoices() {
                   >
                     <motion.div
                       key={slideKey}
-                      className="dash-kpi-grid dash-kpi-4"
+                      className="admin-kpi-grid admin-kpi-4"
                       custom={slideDirection.current}
                       variants={{
                         enter: (dir: number) => ({
@@ -740,11 +740,11 @@ export default function Invoices() {
             animate={{ opacity: 1, y: 0 }}
             transition={{ duration: 0.25, delay: 0.12 }}
           >
-            <div className="offers-tabs mb-6">
+            <div className="admin-tabs mb-6">
               {STATUS_FILTERS.map((f) => (
                 <button
                   key={f.value}
-                  className={`offers-tab ${statusFilter === f.value ? "active" : ""}`}
+                  className={`admin-tab ${statusFilter === f.value ? "active" : ""}`}
                   onClick={() => {
                     setStatusFilter(f.value);
                     setPage(1);