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/app/clients/tools/util/handleTools.js

@@ -327,7 +327,7 @@ const loadTools = async ({
       const { onSearchResults, onGetHighlights } = options?.[Tools.web_search] ?? {};
       requestedTools[tool] = async () => {
         toolContextMap[tool] = `# \`${tool}\`:
-Current Date & Time: ${replaceSpecialVars({ text: '{{iso_datetime}}' })}
+Current Date & Time: ${replaceSpecialVars({ text: '{{iso_datetime}}' })}${options.req?.body?.timezone ? `\nLocal Date & Time: ${replaceSpecialVars({ text: '{{local_datetime}}', timezone: options.req.body.timezone })}` : ''}
 1. **Execute immediately without preface** when using \`${tool}\`.
 2. **After the search, begin with a brief summary** that directly addresses the query without headers or explaining your process.
 3. **Structure your response clearly** using Markdown formatting (Level 2 headers for sections, lists for multiple points, tables for comparisons).

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,
     });
   }