Fix header navigation and active view indication

[singhanurag0317-bit]

Related Issue

Addresses #684

Summary

• connect the header Dashboard, Tasks, and Calendar links to the application's existing view-switching behavior
• replace inert placeholder links with meaningful navigation targets
• synchronize a visible active header state and aria-current for clearer keyboard and screen-reader navigation
• clear the top-level active state when users enter Focus Mode or Archived Tasks from the sidebar

Implementation Note

StudyPlan is a vanilla JavaScript single-page application, so this reuses its existing view state rather than adding a routing dependency. Dashboard restores the planner overview, Tasks shows all tasks, and Calendar opens the calendar view.

Testing

• npm.cmd test
• node --check server.js
• node --experimental-default-type=module --check js/app.js
• git diff --check
• confirmed the application serves the updated header navigation targets locally

Checklist

• Code follows project style
• Tested locally
• No unrelated changes included