refactor(tui): consistent chrome across all views

- Extract divider() and emptyState() helpers to format.go
- All empty states now use bordered box with accent color
- Detail and alert detail panels get header/section dividers
- SLA label width 14→16 to match detail/alert panels
- Logs key hints moved from content to dashboard footer
- History/SLA panels use shared divider helper

internal/tui/tab_users.go

@@ -29,7 +29,7 @@ func fmtKey(key string) string {
 
 func (m Model) viewUsersTab() string {
 	if len(m.users) == 0 {
-		return "\n  No users configured. Press [n] to add one."
+		return m.emptyState("No users configured.", "[n] Add a user")
 	}
 
 	var headers []string