bug: btn fix in navbar for navigation (#684)

[vansh-09]

Related Issue

Closes #684

Summary

Fix header navigation so the top header links (Dashboard / Tasks / Calendar) switch the app view. The header links now call the same view handlers used by the sidebar and update the active sidebar state

Changes Made

-[app.js:1032-1060] Added click handlers for .header-nav a that map link text to views (dashboard → calendar view, tasks → all-tasks view, calendar → calendar view), call [updateSidebarActive(...)] and invoke the appropriate render function.

Testing

• Manual (browser): opened the app and clicked the header links — verified the view switches (Calendar / All Tasks) and the sidebar active class updates.
• Server: ran the app locally (npm install then npm start) and verified behavior in the browser.

Screenshots

N/A

Checklist

• Code follows project style
• Tested locally
• No unrelated changes included
• Documentation updated (if applicable)