[FEAT] Add GitLab integration

[Priyanshu-byte-coder]

Tracking issue for full GitLab integration. Split into smaller pieces — pick one below.

Sub-issues

• [FEAT] GitLab OAuth — sign-in and token storage #9 — GitLab OAuth (sign-in + token storage) medium
• [FEAT] GitLab — contribution graph data #10 — GitLab contribution graph data medium
• [FEAT] GitLab — Merge Request analytics #11 — GitLab Merge Request analytics medium

Work on #9 first — #10 and #11 depend on it.

End Goal

Users can connect both GitHub and GitLab accounts. All dashboard metrics aggregate data from both platforms.

[Shravani-36]

Hi I am interested to work on this issue

[Akanksha-020]

I'd like to work on this — contributing under GSSoC'26