fix(qa): ISSUE-001 — fall back to members.length when usage.user_count is missing

The /subscription endpoint returns usage as {tree_count, session_count_this_month}
without user_count, so the Seats UsageRow rendered as " / ∞" (blank current value).
The TS type declared user_count: number, hiding this API/type drift; the old
card-stack design hid it visually because each stat had its own border. The new
flat layout surfaced the gap.

Owners get a fallback to members.length (already fetched). Non-owners can't
fetch members and don't need seat-count info, so the row hides entirely for
them. Verified live: owner now sees Seats 2 / ∞.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/pages/AccountSettingsPage.tsx

@@ -406,7 +406,12 @@ export function AccountSettingsPage() {
                 current={usage.session_count_this_month}
                 max={limits.max_sessions_per_month}
               />
-              <UsageRow label="Seats" current={usage.user_count} max={limits.max_users} />
+              {(() => {
+                const seatCount = usage.user_count ?? (isAccountOwner ? members.length : null)
+                return seatCount !== null ? (
+                  <UsageRow label="Seats" current={seatCount} max={limits.max_users} />
+                ) : null
+              })()}
             </div>
           ) : (
             <p className="text-sm text-muted-foreground">Plan limits unavailable.</p>