Feat/UI improvement mini navbar

[DevaanshKathuria]

Description

Fixes three pre-existing bugs in the SideBar component group,
all related to the ongoing MUI v4 → v5 migration.

Changes

sidelist.jsx:

• Fixed hardcoded key="menu-items" on every mapped div — every
  item was getting the same key, causing duplicate key warnings
• Migrated from deprecated makeStyles (@mui/styles) to MUI v5
  styled() API
• Fixed children rendering inside MenuList using React.Children.map
  to avoid Fragment warning

SideBar/index.jsx:

• Removed Fragment wrapper (<>...</>) from return statement —
  this was causing "MUI: Menu doesn't accept Fragment as child" warning
• Migrated from deprecated makeStyles to MUI v5 styled() API
• Simplified conditional rendering to return single elements directly

Related Issue

Closes #314

How Has This Been Tested?

• Ran npm run dev — no duplicate key warnings in console
• MUI Fragment warning resolved
• No functional regressions introduced
• App builds and runs correctly on localhost:5173

Types of changes

• Bug fix (non-breaking change which fixes an issue)