fix(web): recover from quiet or stalled connections

[testikun]

Problem

The OpenPI Web event stream could remain silently half-open, and quiet streams gave the browser no bounded way to verify canonical state. API and prompt-admission waits were also unbounded, leaving users without actionable feedback when a connection stalled. This completes the remaining liveness/recovery slice of #363 and is intentionally separate from the state-regression work in #364.

Value

Web clients now detect silent failures, periodically reconcile quiet connections with the canonical snapshot, and retain prompt drafts with explicit localized feedback when admission times out.

Approach

• Send bounded SSE comment heartbeats per connected client without advancing the event cursor, and clean up each timer on disconnect or Host shutdown.
• Use heartbeat cadence for low-frequency, single-flight snapshot reconciliation and a read watchdog for stalled streams.
• Bound ordinary API and prompt-admission requests with AbortController; preserve failed prompt drafts and surface distinct pending, timeout, error, and reconnecting feedback.
• Keep all recovery behavior within the existing Host/event/snapshot lifecycle rather than adding another connection-state store.

Validation

• bun run check — passed.
• bun run test — passed: Node 1244 passed / 1 platform skip; Vitest 30 passed.
• Real local Workbench smoke test in an actual browser: loaded the authenticated UI from the source checkout, stopped the live Host, and observed the localized reconnect-and-resnapshot state. No mock image was used. Screenshot is attached to the first bug(web): 长 workflow/subagent 回合期间 Web UI 卡在"正在准备任务..."数小时不更新，新输入提示词也不显示 #363 claim comment.

Impact

• User-visible behavior: stalled requests and interrupted live updates now produce explicit English/Chinese feedback; failed prompt admission restores the draft.
• Model-visible context/tools: none.
• Runtime/lifecycle: one unref'd heartbeat timer per connected SSE client, bounded by the existing eight-client cap and cleared during disconnect/shutdown.
• Persisted config/data: none.
• Compatibility/risk: protocol-compatible SSE comments; snapshot cursors are unchanged by heartbeats. Reconciliation adds one snapshot request per four quiet heartbeats.