Fix invisible approval requests in dashboard

[CastleOneX]

Summary

• keep a bounded recent approval history instead of dropping resolved or timed-out approvals immediately
• return recent approvals from /api/approvals
• make the dashboard poll and surface pending approvals with a badge/toast so requests do not disappear before the user sees them

Why

Approval-gated actions such as shell_exec could time out or resolve before the Approvals page was opened. In practice this made requests appear to vanish, which made the security flow feel broken even though the request had existed briefly.

Validation

• cargo check -p openfang-kernel -p openfang-api -j 1

[jaberjaber23]

Well done. ApprovalRecord history with bounded VecDeque, pending badge in nav, toast notifications, 5s polling, expired filter. Tests cover all three decision paths. Clean implementation.