fix(validation): enforce chat and server input limits

[nyxsky404]

Summary

• Added shared message and server-name validation limits across the frontend and backend.
• Trim and reject empty or over-limit content for server messages, direct messages, message edits, and server creation.
• Added MongoDB maxlength safeguards, legacy Socket.IO validation hardening, and a focused validation unit script.

Fixes #135

Type of Change

• Bug fix
• Feature
• Documentation
• Refactor
• Tooling / developer experience

Validation

• cd frontend && npm run lint
• cd frontend && npm run build
• Server starts with cd server && npm start, if backend code changed
• I checked the feature or page I changed in the app

Additional checks:

• cd server && npm run test:validation
• cd server && node --check src/routes/chat.js
• cd server && node --check src/routes/directMessages.js
• cd server && node --check src/routes/servers.js
• cd server && node --check src/socket/index.js

Screenshots or Recording

Not included. The UI changes are character counters and validation messages on existing inputs.

Notes for Reviewers

• Message content is capped at 2000 characters.
• Server names are capped at 100 characters.
• Whitespace-only values are rejected after trimming.
• npm start was not run locally because this workspace does not have server/.env / MONGO_URI configured.

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