fix(test-activity): prevent tooltip overflow on small viewports (@Mubashir21)

[Mubashir21]

Description

Bug: On small/mobile viewports, the activity calendar tooltips near the left and right edges get clipped by the viewport boundary because the tooltip position was hardcoded to "up" for every cell.

Fix: After the cells are rendered, I measure the actual tooltip width using a hidden probe element with the same font styles as the balloon tooltip. For each cell, I use getBoundingClientRect() to check if the tooltip would overflow the left or right edge of the viewport. If it would, the position is switched to "up-left" or "up-right" accordingly. A ResizeObserver on the container recalculates positions on every resize, so it works correctly at any viewport width.

Steps to reproduce: Go to /account, shrink the browser window to mobile width, hover over cells on the left or right edge of the activity calendar: the tooltip gets cut off.

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