feat(card): add contributions area chart card

New 5-contributions.svg renders the last year's contribution calendar
as a monthly smooth-filled area chart. Pure Go SVG; no extra API calls
— one additional contributionCalendar.weeks block in the existing
profile GraphQL query carries the data.

- Y-axis mirrored on both sides with nice ticks.
- X-axis labels in YY/MM format, every other month to avoid overlap.
- Smooth curve via Catmull-Rom interpolation converted to cubic Bezier
  (d3.curveCatmullRom default tension 0.5).
- Missing months between first and last are inserted as zero-count so
  the chart stays time-continuous.

internal/github/queries.go

@@ -30,7 +30,15 @@ query($login: String!, $after: String) {
       totalPullRequestReviewContributions
       totalRepositoryContributions
       restrictedContributionsCount
-      contributionCalendar { totalContributions }
+      contributionCalendar {
+        totalContributions
+        weeks {
+          contributionDays {
+            contributionCount
+            date
+          }
+        }
+      }
     }
     repositories(
       first: 100