Reconnect SSE stream after fatal EventSource errors

[selfcontained]

Problem

EventSource only auto-reconnects after transient failures. A fatal one — a non-200 response or a wrong content-type, e.g. loading the page while the server is mid-restart — puts it in CLOSED permanently, per spec.

use-sse.ts didn't handle that case:

• onerror only called recordSSEReconnect().
• The dead instance stayed in eventSourceRef, so openSSE() early-returned forever.
• Only a tab hide/show or an authState change cleared the ref — a visible desktop tab never self-healed.

Impact: silent loss of all realtime updates (agent status, terminal-state banner, media/review refreshes, injection-hold badge) while the app looks perfectly healthy.

Fix

In onerror, branch on readyState:

• CONNECTING — the browser is retrying on its own, leave it alone.
• CLOSED — the instance is dead: drop it from the ref and reopen on a capped backoff (1s, doubling to 30s).

The backoff resets when the stream delivers an event (the server sends a snapshot on every connect, so that's a clean success signal) and when the tab is foregrounded.

No heartbeat watchdog. The idea suggested one as optional; it isn't implementable client-side here. The server's keepalives are SSE comments (: keepalive), which EventSource never surfaces to onmessage — a liveness timer couldn't tell a hung proxy from a genuinely idle stream. That would need a real data-event heartbeat from the server; out of scope for this fix.

Validation

Unit — 5 new tests in use-sse.test.ts drive a fake EventSource with fake timers: fatal error reopens, transient error doesn't, backoff doubles, delivery resets the backoff, unmount stops retrying. 3 of the 5 fail against the pre-fix hook (verified by stashing the change).

Live, in a browser — intercepted /api/v1/events with a 500 to force the fatal path:

• Retry timeline while the endpoint was down: +0.3s, +1.4s, +3.5s, +7.7s, +15.8s — the capped doubling, reconnecting on its own with no tab switch.
• End-to-end: renamed an agent server-side while the stream was dead → UI did not update (the bug's symptom); un-blocked the endpoint → UI picked up the new name 2s later, no reload, no tab switch.

Checks — pnpm run check 0 errors · finalize:web builds · web unit 710 passed · server unit 2503 passed · E2E 178 passed / 12 skipped (terminal-live, needs tmux).

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