feat(pr): show total lines of code changed in PR Analytics widget

[anuragbraveboy-sudo]

Description

This PR resolves issue #2450 by adding a "Lines Changed" stat tile to the PR Analytics widget, displaying total additions and deletions across merged PRs in the last 30 days.

Closes #2450

Key Changes

1. GraphQL API Query Optimization:

   • Extended /api/metrics/prs route to run parallel GraphQL search queries for reviews and authored PRs in a single HTTP request.
   • Aggregates and sums additions and deletions for the user's PRs merged in the last 30 days.
   • Added additions/deletions support for target account aggregation (targetAccountId === "combined") and returned 0 fallbacks for GitLab.

2. Frontend Stat Tile:

   • Rendered a new "Lines Changed" stat card displaying additions and deletions with green (+) and red (-) coloring, matching GitHub's diff formatting style (e.g., +1,240 / -380).
   • Adapted the grid layout and loading skeletons to support 7 columns cleanly.
   • Configured filter matching so the tile appears on the "All" and "Merged" filters but is hidden on the "Open" filter.

3. E2E Test Mocks:

   • Updated mock API responses in e2e/helpers/dashboard-mocks.js to return mock additions/deletions, ensuring layout rendering is covered under tests.

Verification

• Run TypeScript compile check: npx tsc --noEmit (Passed).
• Run Playwright E2E tests: npx playwright test e2e/dashboard-widgets.spec.js (3/3 tests passed successfully).

[github-actions]

GSSoC Label Checklist 🏷️

@Umbrella-io — please apply the appropriate labels before merging:

Difficulty (pick one):

• level:beginner — 20 pts
• level:intermediate — 35 pts
• level:advanced — 55 pts
• level:critical — 80 pts

Quality (optional):

• quality:clean — ×1.2 multiplier
• quality:exceptional — ×1.5 multiplier

Validation (required to score):

• gssoc:approved — counts for points
• gssoc:invalid / gssoc:spam / gssoc:ai-slop — does not score

Type labels (type:*) are auto-detected from files and title. Review and adjust if needed.
Points formula: (difficulty × quality_multiplier) + type_bonus