test(qa): chat tab selector handles unread badge suffix

The new topbar badge on the Chat tab (chatThreadCount > 0) broke the
old /^\\s*Chat\\s*$/ regex. Switched to a Playwright filter chain:
.tab filtered by hasText 'Chat' AND hasNotText 'Assistant' (which also
contains 'Chat' substring elsewhere in DOM). All 4 QA suites green:

- 36/36 dogfood (chat_sidebar threads=8 previews=8 times=8)
- 12/12 buyer-script (Approvals action disabled? true)
- 8/8 idle-poll (0 reqs at 30s on every scene)
- 14/14 mobile (390px clean)

qa/full_dogfood.mjs

@@ -262,7 +262,8 @@ record("llm_unconfigured_falls_back_gracefully", unmatchedReply.length > 0 && !/
 // 17. CHAT polish: open Chat tab; if any thread exists, sidebar shows relative
 //     timestamps and last-message previews; unread aggregate banner shows when
 //     other personas have written since last open.
-await p.locator(".tab", { hasText: /^\s*Chat\s*$/ }).click();
+const chatTab = p.locator(".tab").filter({ hasText: "Chat" }).filter({ hasNotText: "Assistant" }).first();
+await chatTab.click({ timeout: 15000 });
 await p.waitForTimeout(2500);
 const initialThreadCount = await p.locator(".chat-thread-row").count();
 if (initialThreadCount === 0) {