feat(ai-chat): rephrase login title to action-led copy

AI_CHAT_LOGIN_TITLE: 'Sign In to Use AI' → 'Sign in to get started with AI'.
Used by the new login UI which reuses the placeholder card layout, so
the title pairs with the placeholder header to read as an inviting
call-to-action rather than a gating message.

Author: abose

src/nls/root/strings.js

@@ -2180,7 +2180,7 @@ define({
     "AI_CHAT_PLACEHOLDER_CARD_BUGS_DESC": "Scan for issues",
     "AI_CHAT_PLACEHOLDER_CARD_IMPROVE_TITLE": "Suggest improvements",
     "AI_CHAT_PLACEHOLDER_CARD_IMPROVE_DESC": "UX & performance tips",
-    "AI_CHAT_LOGIN_TITLE": "Sign In to Use AI",
+    "AI_CHAT_LOGIN_TITLE": "Sign in to get started with AI",
     "AI_CHAT_LOGIN_MESSAGE": "Sign in to your {APP_NAME} account to access AI features.",
     "AI_CHAT_LOGIN_BTN": "Sign In",
     "AI_CHAT_INTRO_GUEST_TITLE": "Design layouts, fix bugs, and build faster with AI",