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/ProjectCreate.tsx

@@ -284,7 +284,7 @@ export default function ProjectCreate() {
             <div className="admin-form-row">
               <FormField label="Zákazník" error={errors.customer_id} required>
                 {form.customer_id ? (
-                  <div className="offers-customer-selected">
+                  <div className="admin-customer-selected">
                     <span>{form.customer_name}</span>
                     <button
                       type="button"
@@ -308,7 +308,7 @@ export default function ProjectCreate() {
                   </div>
                 ) : (
                   <div
-                    className="offers-customer-select"
+                    className="admin-customer-select"
                     onClick={(e) => e.stopPropagation()}
                   >
                     <input
@@ -323,16 +323,16 @@ export default function ProjectCreate() {
                       placeholder="Hledat zákazníka..."
                     />
                     {showCustomerDropdown && (
-                      <div className="offers-customer-dropdown">
+                      <div className="admin-customer-dropdown">
                         {filteredCustomers.length === 0 ? (
-                          <div className="offers-customer-dropdown-empty">
+                          <div className="admin-customer-dropdown-empty">
                             Žádní zákazníci
                           </div>
                         ) : (
                           filteredCustomers.slice(0, 20).map((c) => (
                             <div
                               key={c.id}
-                              className="offers-customer-dropdown-item"
+                              className="admin-customer-dropdown-item"
                               onMouseDown={() => selectCustomer(c)}
                             >
                               <div>{c.name}</div>