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/CompanySettings.tsx

@@ -515,7 +515,7 @@ export default function CompanySettings({
         </motion.div>
       )}
 
-      <div className="offers-settings-grid">
+      <div className="admin-settings-grid">
         {/* Company Info */}
         <motion.div
           className="admin-card"
@@ -1080,7 +1080,7 @@ export default function CompanySettings({
           </div>
           <div className="admin-card-body">
             <div className="admin-form-row">
-              <div className="offers-logo-section">
+              <div className="admin-logo-section">
                 <label
                   className="admin-form-label"
                   style={{ display: "block", marginBottom: 4 }}
@@ -1088,7 +1088,7 @@ export default function CompanySettings({
                   Logo (světlý režim)
                 </label>
                 {logoUrl && (
-                  <div className="offers-logo-preview">
+                  <div className="admin-logo-preview">
                     <img src={logoUrl} alt="Logo (světlý režim)" />
                   </div>
                 )}
@@ -1130,7 +1130,7 @@ export default function CompanySettings({
                   PNG, JPEG, GIF nebo WebP, max 5 MB
                 </small>
               </div>
-              <div className="offers-logo-section">
+              <div className="admin-logo-section">
                 <label
                   className="admin-form-label"
                   style={{ display: "block", marginBottom: 4 }}
@@ -1138,7 +1138,7 @@ export default function CompanySettings({
                   Logo (tmavý režim)
                 </label>
                 {logoUrlDark && (
-                  <div className="offers-logo-preview">
+                  <div className="admin-logo-preview">
                     <img src={logoUrlDark} alt="Logo (tmavý režim)" />
                   </div>
                 )}