fix(web): show recent session times in local timezone

[lwylab]

Summary

• Home recent-session cards stripped the Z from UTC ISO timestamps and printed the digits as wall-clock time, so users in China (UTC+8) saw times ~8 hours behind.
• Parse as an absolute Instant and format with toLocaleString using the browser local timezone; UI language only selects locale style (zh-CN / en), not a hard-coded geographic zone.

Note

Replaces closed PR #227 — GitHub refused to reopen it because lwylab/OpenKB is a private non-fork and head refs are not readable. This PR is opened from a fresh fork lwylab/OpenKB-1.

Test plan

• Open Workbench Home with at least one recent chat session
• Confirm the card timestamp matches local clock (not UTC / ~8h behind in China)
• Switch UI language between zh and en; timezone should stay local, only date/number style may change
• Session with missing/invalid updated_at still renders without crashing