Speed up web terminal history open

[edwin-zvs]

Summary

• reduce web terminal initial PTY replay from 1 MiB to 128 KiB for faster first paint
• cap each older-history fetch/replay chunk at 1 MiB instead of growing to 8 MiB
• add a visible history action when older PTY bytes are available, while preserving top-scroll lazy loading
• keep the history action disabled during older-history replay and hide it when no older bytes remain
• preserve replay range metadata even when a replay response has no payload bytes

Tests

• cargo build
• cargo test -p agentd-e2e web_client_loads_and_websocket_connects --test web_smoke -- --nocapture

Notes

No recording: this is a performance/behavior change covered by e2e browser assertions. The PR changes embedded web UI assets, so the relevant binary is agentd.