[codex] Show full session update datetime

[boxp]

What changed

• changed session updated time display from HH:mm:ss to yyyy-MM-dd HH:mm:ss
• adjusted the auto table/card switch threshold to account for the wider datetime column
• added tests for full datetime rendering and the updated compact-mode threshold

Why

• showing only the time makes it hard to understand recency once sessions cross midnight
• the previous table width assumption became too narrow after expanding the timestamp display

Impact

• session rows and cards now show full local datetime for :last-updated
• auto display mode falls back to card view before the widened table layout wraps

Root cause

• the view layer formatted timestamps as time-only, so the UI lost the date context even though sorting still used the full ISO timestamp internally

Validation

• make ci