refactor: design critique fixes for account pages

- Admin accounts: replace dense card grid with compact DataTable
- Account settings: remove redundant hero card, stat grid, header pills
- Fix bg-accent (orange) misuse on decorative elements across 7 files
- Add ConfirmButton for destructive actions (deactivate, remove member)
- Replace single-field modals with inline editing (plan, trial)
- Add contextual help: display code tooltip, improved empty states
- Non-owner aside explanation for hidden owner-only sections
- Admin sidebar: group 11 items into 5 labeled sections
- Rename UsersPage.tsx → AccountsPage.tsx to match route
- Fix border radius consistency, hide zero-count badges

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

frontend/src/components/admin/DataTable.tsx

@@ -53,7 +53,7 @@ export function DataTable<T>({
     <div className="overflow-x-auto rounded-lg border border-border">
       <table className="w-full text-sm">
         <thead>
-          <tr className="border-b border-border bg-accent">
+          <tr className="border-b border-border bg-elevated">
             {columns.map((col) => (
               <th
                 key={col.key}
@@ -90,7 +90,7 @@ export function DataTable<T>({
               <tr key={i} className="border-b border-border last:border-0">
                 {columns.map((col) => (
                   <td key={col.key} className="px-4 py-3">
-                    <div className="h-4 w-3/4 animate-pulse rounded bg-accent" />
+                    <div className="h-4 w-3/4 animate-pulse rounded bg-muted" />
                   </td>
                 ))}
               </tr>
@@ -107,7 +107,7 @@ export function DataTable<T>({
             data.map((item) => (
               <tr
                 key={keyExtractor(item)}
-                className="border-b border-border last:border-0 hover:bg-accent transition-colors"
+                className="border-b border-border last:border-0 hover:bg-elevated transition-colors"
               >
                 {columns.map((col) => (
                   <td key={col.key} className={cn('px-4 py-3', col.className)}>