fix(i18n): pluralize the collapsed subagent-count badge

The agent-tree "N subagent" badge on the Dashboard rendered
t("common:subagent", { count }) — but common:subagent is a flat word,
not an i18next plural key (no _one/_other), so the count never selected a
plural and it always read "2 subagent", "5 subagent", etc.

Use common:subagent_label (which already has _one/_other and embeds the
count) — the same key SessionDetail's badge uses — so both surfaces read
"1 subagent" / "2 subagents" consistently. Add an i18n test locking the
plural forms for common:subagent_label and kanban:session.subagentSummary.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: hoangsonww

client/src/i18n/__tests__/i18n.test.ts

@@ -32,4 +32,17 @@ describe("i18n resources", () => {
     expect(i18n.resolvedLanguage?.startsWith("vi")).toBe(true);
     expect(i18n.t("nav:dashboard")).toBe("Tổng quan");
   });
+
+  it("pluralizes the subagent count labels in English", async () => {
+    await i18n.changeLanguage("en");
+    // The collapsed agent-tree badge (Dashboard) and SessionDetail both render
+    // this key with a count. It MUST use i18next plural forms (_one/_other) so
+    // "2 subagent" never shows — the flat common:subagent word is not a plural
+    // key and rendering it with a count is the bug this guards against.
+    expect(i18n.t("common:subagent_label", { count: 1 })).toBe("1 subagent");
+    expect(i18n.t("common:subagent_label", { count: 2 })).toBe("2 subagents");
+    // The main-agent card subtitle carries its own kanban plural key.
+    expect(i18n.t("kanban:session.subagentSummary", { count: 1 })).toBe("1 subagent");
+    expect(i18n.t("kanban:session.subagentSummary", { count: 3 })).toBe("3 subagents");
+  });
 });

client/src/pages/Dashboard.tsx

@@ -1310,7 +1310,7 @@ export function Dashboard() {
                               }
                               className="ml-7 mt-1 text-[11px] text-violet-400 hover:text-violet-300 transition-colors"
                             >
-                              {totalDesc} {t("common:subagent", { count: totalDesc })}
+                              {t("common:subagent_label", { count: totalDesc })}
                               {activeDesc > 0 && (
                                 <span className="text-emerald-400 ml-1">
                                   ({activeDesc} {t("common:active")})