refactor: centralize auth-gate strings into src/lib/constants.ts

[nishupr]

Summary

Implements #750 — creates src/lib/constants.ts and extracts scattered hardcoded strings into an exported object for cleaner maintenance.

Scope

Audited the codebase for hardcoded alert()/message strings (16 files use alert() directly). The clearest, most valuable duplication was the "please log in to do X" family of messages, which appeared — with slightly different wording each time — across 8 different files. Centralized those first, since they're genuinely the same concept repeated, not just coincidentally similar text.

Other one-off success/error strings (e.g. "User updated successfully.", "Failed to update user.") are more page-specific and less clearly duplicated; left them as-is for a possible follow-up rather than making this PR sprawl across all 16 files.

Changes

New: src/lib/constants.ts

export const AUTH_MESSAGES = {
  LOGIN_TO_STAR_PROJECTS: 'Please login to star projects.',
  LOGIN_TO_STAR_RESOURCES: 'Please login to star resources!',
  LOGIN_TO_REACT: 'Please login to react.',
  LOGIN_TO_LIKE_PROJECTS: 'Please login to like projects.',
  LOGIN_TO_FOLLOW_USERS: 'Please login to follow users.',
  LOGIN_TO_CONNECT_GITHUB: 'Please login to connect your GitHub account.',
  LOGIN_TO_START_DISCUSSION: 'Please login to start a discussion.',
  LOGIN_TO_REPLY_TO_DISCUSSION: 'Please login to reply to this discussion.',
  LOGIN_TO_VIEW_PROFILE: 'Please login to view your profile.',
} as const;

Migrated 8 files to use AUTH_MESSAGES instead of literal strings:

• src/components/projects/ProjectCard.tsx
• src/components/profile/FollowButton.tsx
• src/components/profile/UserProfile.tsx
• src/components/resources/InternshipCalendarModal.tsx
• src/app/community/page.tsx
• src/app/community/view/client.tsx
• src/app/opensource/page.tsx
• src/app/u/client.tsx

No behavior or copy changes — every message renders exactly as before, just sourced from one shared file instead of being duplicated inline.

How to test

1. npm run dev
2. While logged out, try to star a project, follow a user, and react to a discussion
3. Confirm each alert/message text is unchanged from before

Related Issue

Closes #750

[nishupr]

hey @Aditya948351 thank you for merging the PR, could you please add the gssoc:approved label as well!