feat(replay): interactive session replay with timeline scrubber

[Mukller]

Summary

Closes #2 — adds a full Session Replay page at /sessions/:id/replay.

Features

• Timeline scrubber — <input type="range"> that spans the entire event sequence; dragging or using arrow keys steps through events deterministically
• Play / Pause with four speed settings: 0.5×, 1×, 2×, 5× (interval-based, restarts cleanly on speed change while playing)
• Step controls — step back / forward one event at a time, jump to start / end
• Lifecycle mini-map — compact visual bar showing SessionStart, Stop, SubagentStop, Compaction, and APIError markers; click any marker to jump there instantly
• Agent state panel — shows the derived status (working / waiting / completed / error) of every agent seen up to the current cursor position
• Windowed event list — renders only ±50 events around the cursor, so sessions with 10 000+ events stay smooth
• Keyboard shortcuts: Space play/pause · ←/→ step · Home/End jump to start/end (ignored when focus is in a form field)

Implementation notes

• Deterministic state engine — events sorted by created_at then id tie-break; applyEvent maps event_type → AgentStatus and accumulates per-agent state
• Checkpoints every 100 events — seekToIndex jumps to the nearest checkpoint then replays the tail, keeping seek cost O(checkpoint_interval) instead of O(n)
• replay i18n namespace — full English locale; falls back to en for other languages (zh/vi/ko) until community translations land
• detail.replay key added to all four existing sessions.json locales; "Replay" button added to the SessionDetail header bar

Test plan

• Open any completed session → click Replay button in the header
• Press Play — events step forward at the chosen speed
• Drag the scrubber to an arbitrary position — agent states update instantly
• Click a lifecycle marker on the mini-map — cursor jumps there
• Press Space, ←, →, Home, End — all work without focus issues
• Open a session with > 1 000 events — no UI lockup during playback
• cd client && npx tsc --noEmit — zero errors

[Dcastroro]

The replay cursor and playback state are not reset when the route id changes or when a new event set is loaded. React can reuse this page instance when navigating between sessions, so a cursor from a long session may remain beyond the end of a shorter session. That leaves currentEvent null, the range value outside its new max, and reconstructed state at an unrelated checkpoint until the user manually jumps or presses play. Reset cursor to zero and stop playback when id changes or immediately before applying the new session payload, and add a navigation test from a longer session to a shorter one.