Add local timezone datetime support in agent prompts

- Add timezone support to replaceSpecialVars function with dayjs timezone plugin
- Add new special variables: {{local_datetime}} and {{local_date}}
- Update specialVariables config to include new variables
- Pass timezone from client via Intl.DateTimeFormat API
- Add timezone field to TPayload type
- Update all client-side replaceSpecialVars calls to include timezone
- Add comprehensive tests for timezone-aware special variables
- Update web_search tool context to include local datetime when timezone available

Co-authored-by: berry-13 <81851188+berry-13@users.noreply.github.com>

api/server/services/Endpoints/agents/agent.js

@@ -185,6 +185,7 @@ const initializeAgent = async ({
     agent.instructions = replaceSpecialVars({
       text: agent.instructions,
       user: req.user,
+      timezone: req.body?.timezone,
     });
   }