feat: User-controlled session TTL — presets + custom duration

[OneStepAt4time]

Summary

Replace the global maxSessionAgeMs with per-session TTL chosen by the user at creation time.

Design

• Presets: 1h, 2h (default), 4h, 8h, 24h
• Custom: any duration via text input
• Session reaper respects per-session expiresAt instead of global max
• Expired sessions are archived (transcript + metadata preserved), not deleted
• TTL countdown shown in session header bar

API Changes

• POST /v1/sessions gains ttlMs field (optional, default 7200000)
• Session reaper checks expiresAt = createdAt + ttlMs per session

UI

• TTLPicker component in CreateSessionModal
• Preset buttons + custom input field
• TTL countdown in session header

Depends On

• Session persistence issue (needs expiresAt field in stored sessions)

Spec

See docs/superpowers/specs/2026-03-28-dashboard-ux-p1-design.md — Section 2: User-controlled TTL