Center timeline connector line on entry icons

[AsmitNepali]

What

Re-centers the vertical connector line in the activity-log timeline so it runs through the middle of each entry's icon, with even gaps above and below every icon.

Why

The connector was positioned with a hardcoded left-[22px] on a 1px-wide line. Anchoring a 1px line at the icon's center coordinate draws it half a pixel to the right of the axis, so on most displays it visibly leaned off-center. The line also ran from top-9 to each row's bottom-0, leaving a small gap below an icon but a larger gap above the next one.

How

• Wrap the 1px line in a flex container sized to the icon column (left-2 matches the entry's px-2, w-7 matches the 28px icon grid column) and center it with justify-center — no pixel math, structurally centered on the true 22px axis.
• Extend the line into the next row (-bottom-2) so the gap above and below each icon is symmetric.
• Uses only standard Tailwind utilities (no arbitrary values), so it compiles reliably in consumer themes.

Applied to both the grouped and ungrouped timeline branches. All 46 tests pass.