fix(heatmap): prevent activity tooltip clipping at edges (@kyroskoh)

[kyroskoh]

Summary

• Reposition activity heatmap balloons near calendar edges (up-left / up-right) so tooltips stay in viewport
• Fixes clipping of test-count tooltips on rightmost dates (e.g. today)
• Adds unit tests for balloon position selection

Fixes#8336

Test plan

• Open account/profile page with yearly activity heatmap (last 12 months)
• Hover rightmost cells (including today) — full tooltip visible, not clipped
• Hover leftmost cells — tooltip still fully readable
• Hover middle cells — centered tooltip still looks correct
• pnpm vitest run frontend/__tests__/elements/test-activity.spec.ts

[fehmer]

duplicate #8324