[Feature] - Add image upload functionality with multer and integrate in editor

[ElizabethSobiya]

Description

• Image upload, toolbar button uploads to /api/uploads, inserts as
  ImageNode (block-level DecoratorNode). Click to select,
  Backspace/Delete to remove. Persists after refresh. Included in PDF
  export.
• Auto-save indicator fix "Saving…" state was never visible because
  localStorage is sync and React batched the state updates. Fixed with a
  deferred setIsSaving(false).
• ESLint v9 flat config, added root eslint.config.mjs so lint-staged
  works from repo root. Added missing eslint plugins to apps/web.
• Vite dev proxy, proxies /api and /uploads to localhost:3001,
  eliminating CORS in development.
• CI fix npm ci → npm install to fix cross-platform lock file
  failures on Linux runners.
• Docs README and CONTRIBUTING updated with image upload, quick-start
  improvements, and common error fixes.

Type of change

• Bug fix
• New feature
• Breaking change
• Docs / chore

How to test

Screenshots

Checklist

• Tests pass
• No TypeScript errors (npm run typecheck)
• No lint warnings (npm run lint)
• Docs updated if needed
• Targets the dev branch (not main)