feat: Add Document Chunking Sandbox — Issue #164

[Arjita-2B-18]

Summary

Adds an interactive chunking sandbox at /sandbox so contributors and
users can preview how text will be split before running the full
ingestion pipeline.

Changes

Frontend

• src/App.tsx — Added /sandbox as a ProtectedRoute
• src/pages/Sandbox.tsx — New page with:
  • Text paste area with live character counter
  • Sliders for chunk size (50–2000) and overlap (auto-clamped to chunkSize - 10)
  • "Preview Chunks" button calling POST /api/chunk-preview
  • Result cards with per-chunk character count
  • Purple highlight on the overlap region in each chunk (except the first)
  • Stats pills: total chunks / size / overlap
  • Reset button, full error + loading states
  • Matches existing design system: bg-[#0b0b0f], Sidebar, accent-blue, accent-purple

Backend

• backend/controllers/chat.controller.js
  • Added isolated chunkText() utility function (no UI coupling, reusable by worker)
  • Added chunkPreview handler using asyncHandler + ApiResponse/ApiError
  • Input validation: required string, max 100k chars, chunkSize 10–5000, overlap ≥ 0
  • Zero DB writes. Zero vector operations. Zero production chat data created.
• backend/routes/chat.routes.js — Registered POST /chunk-preview

Acceptance Criteria

• Users can paste text and choose chunking parameters
• Page shows resulting chunks with counts and overlap behavior highlighted
• Sandbox does not store vectors or create production chat data

Notes

• chunkText() is intentionally isolated in both files so the ingestion
  worker can import the backend version directly in a future PR
• No new npm dependencies added

Closes #164

[avishek0769]

@Arjita-2B-18 resolve merge conflicts