feat(placement-cell): Add full Placement Cell module with role-based UI

[vikrantwiz02]

Summary

• Adds a complete Placement Cell frontend module under src/Modules/PlacementCell/ with role-based tab views for Student, Placement Officer, Placement Chairman, and Dean/Faculty
• UI is built to match the existing production design system — uses shared ModuleTabs (caret-arrow scrollable tabs) and CustomBreadcrumbs exactly as used in the Academics and Programme Curriculum modules
• Table styling matches production: blue header (#C5E2F6 / #3498db), alternating row colors (#E6F7FF / #fff), rounded bordered container

Features by role

Student

• Dashboard: active eligible job cards with deadline badges (red/orange/green), announcements, live CPI card
• Job Openings: browse and apply; external apply-link support (tracks application in DB and opens external portal)
• My Applications: color-coded status badges (applied / shortlisted / placed / rejected / withdrawn), withdraw button
• My Profile: resume URL, LinkedIn, GitHub links; opted-out is one-way (shows permanent alert once set, cannot revert via portal)

Placement Officer

• Students: batch select → table of students with live published CPI, placement status (On-Campus / Off-Campus / Placed / Not Placed), CPI filter (min/max), Export to XLSX/PDF; per-row Actions menu to mark placed or set opted-out
• Off-Campus: record and delete off-campus placements (students automatically marked as placed in statistics)
• Companies: full CRUD for company records
• Job Posts: create/edit/activate/deactivate job posts; optional CTC/Stipend; optional external apply link; per-job applicant list with bulk status update
• Announcements: create (with pin option) and delete announcements
• Statistics: batch-wise live stats — total students, placed, on-campus vs off-campus breakdown, companies visited, avg/highest CTC
• Export: download placement data as XLSX or PDF

Placement Chairman — Students, Statistics, Export (read + export only)

Dean/Faculty — Statistics, Announcements (read-only)

Other changes

• src/components/sidebarContent.jsx: fixed sidebar URL from "/" to "/placement-cell"
• src/routes/placementRoutes/index.jsx: all API route constants
• src/App.jsx: registered /placement-cell route

Test plan

• Login as student → verify Dashboard, Job Openings, My Applications, My Profile tabs appear with caret arrows
• Login as placement officer → verify all 7 tabs appear; create a job post; apply as student; update status in Applicants
• Add an off-campus placement → verify student appears as "Off-Campus" in Students tab
• Set opted-out as student → verify it cannot be reverted via the portal
• Apply via external link → verify application is tracked in DB and external URL opens
• Statistics tab → select a batch → verify live counts appear
• Login as dean → verify only Statistics and Announcements tabs appear