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/components/dashboard/DashSessions.tsx

@@ -218,17 +218,17 @@ export default function DashSessions() {
             </div>
           )}
           {!sessionsLoading && sessions.length > 0 && (
-            <div className="sessions-list">
+            <div className="dash-sessions-list">
               {sessions.map((session) => (
                 <div
                   key={session.id}
-                  className={`session-item ${session.is_current ? "session-item-current" : ""}`}
+                  className={`dash-session-item ${session.is_current ? "dash-session-item-current" : ""}`}
                 >
-                  <div className="session-icon">
+                  <div className="dash-session-icon">
                     {getDeviceIcon(session.device_info?.icon)}
                   </div>
-                  <div className="session-info">
-                    <div className="session-device">
+                  <div className="dash-session-info">
+                    <div className="dash-session-device">
                       {session.device_info?.browser} na{" "}
                       {session.device_info?.os}
                       {session.is_current && (
@@ -240,13 +240,13 @@ export default function DashSessions() {
                         </span>
                       )}
                     </div>
-                    <div className="session-meta">
+                    <div className="dash-session-meta">
                       <span>{session.ip_address}</span>
-                      <span className="session-meta-separator">|</span>
+                      <span className="dash-session-meta-separator">|</span>
                       <span>{formatSessionDate(session.created_at)}</span>
                     </div>
                   </div>
-                  <div className="session-actions">
+                  <div className="dash-session-actions">
                     {!session.is_current && (
                       <button
                         onClick={() =>