Finish session architecture with heartbeat, routing, and tap-to-resend

[Andyccr]

Completes the architecture split started in #6 and #8. ChatSession is now an orchestrator: heartbeat, payload routing, and status copy live in their own modules, RoomManager no longer leaks the session, and failed self messages can be tapped to resend on the existing v1 wire.

What changed

• Heartbeat owns hello / relay-poll timers and visibility pause.
• Payload router decodes v1 hello / typing / ack / chat; the session only applies effects.
• Status is derived from the session machine (deriveStatus + sameStatus).
• Outbound keeps the original payload after ack timeout so a late ack or tap-to-resend can settle.
• Presence prunes peers that stop announcing (lastSeenAt).
• Cache drops in-flight pending markers but keeps failed so reload can retry.
• RoomManager exposes isJoined() / resend() instead of getSession().
• UI: self lines show 发送中 / 未送达; tapping 未送达 resends the same id.

Still static GitHub Pages, public WebTorrent/Nostr signalling, no TURN, no libp2p, no PWA, no protocol v2, no restyle.

Test plan

• npm test (53 tests)
• npm run build:pages
• Two windows, same room: send, see 发送中 then 送达 when the peer supports ack
• Force a failed delivery (or wait for timeout with no ack-capable peer gone), tap 未送达 to resend
• Switch rooms / reload: pending markers are not restored; failed lines remain retryable

CI on this branch is green. Local two-window check on the Vite app confirmed P2P join, bidirectional chat, and the 送达 mark on self bubbles.