dashboard: SharedWorker for SSE multiplexing (closes #448)

mara on #448: "firefox disconnects bc of too many tabs. needs bg
service worker". picked SharedWorker over full Service Worker:
smaller change, addresses the actual problem (shared connection
across tabs), no offline-cache scope creep.

architecture

per-tab `new EventSource('/dashboard/stream')` replaced with a
SharedWorker-backed facade. one SharedWorker instance per origin
holds ONE upstream EventSource and fans every server-sent event
out to every connected tab via MessagePort. N hyperhive tabs now
share ONE backend connection, immune to Firefox's per-tab SSE
throttling under many-open-tabs pressure.

wire protocol (port.postMessage):

  tab → worker
    { kind: 'subscribe',   url: '/dashboard/stream' }
    { kind: 'unsubscribe', url: '/dashboard/stream' }

  worker → tab
    { kind: 'open',    url }
    { kind: 'message', url, data: '<raw SSE data>' }
    { kind: 'error',   url }

subscription tracking is per (port, url). a late subscriber that
joins after the upstream is already OPEN gets a synthetic 'open'
event so its onStreamOpen handler still runs (triggers the
snapshot re-sync that recovers events lost during the join gap).
unsubscribing the last port for a URL closes the upstream
EventSource so we don't leak idle streams.

files

- frontend/packages/dashboard/src/stream-worker.js: new — the
  worker. multi-URL multiplexing via Map<url, {es, ports}>.
- frontend/packages/dashboard/src/common.js: new exported helper
  openStream(url) — returns an EventSource-shaped facade backed
  by the SharedWorker. graceful fallback to direct EventSource
  when SharedWorker is unavailable.
- frontend/packages/dashboard/src/app.js: replaces the inline
  new EventSource('/dashboard/stream') with openStream.
- frontend/packages/dashboard/src/flow.js: passes
  streamFactory: openStream to termCreate so the broker
  terminal's SSE goes through the worker too.
- frontend/packages/shared/src/terminal.js: accepts an optional
  streamFactory(url) option. default unchanged — non-dashboard
  consumers (per-agent UI) keep using direct EventSource.
- frontend/packages/dashboard/build.mjs: new esbuild entry for
  stream-worker.js → dist/static/stream-worker.js (separate
  bundle because SharedWorker scripts run in a different global
  scope and can't be inlined into app.js).

scope kept tight

- per-agent UI's /events/stream stays on direct EventSource. the
  agent UI's tab count per agent is typically 1; SharedWorker
  helps when you have N tabs hitting the SAME stream and the
  per-agent stream URLs differ. if mara wants the agent UI to
  share its workers too it's a separate small PR.
- no offline-cache, no push notifications — those need full
  Service Worker; explicit non-goal here per the design Q.

validation

- npm run build --workspace=@hive/dashboard clean.
- stream-worker.js bundle: 1.8 kb.
- app.js: 154 kb → 158 kb. flow.js: 29.9 kb → 32 kb.
- browser smoke test isn't possible from inside iris's container;
  the EventSource-shaped facade preserves the exact onmessage /
  onopen / onerror surface the existing IIFE consumers use.

frontend/packages/dashboard/build.mjs

@@ -45,6 +45,24 @@ await build({
   logLevel: 'info',
 });
 
+// Stream-worker entry (#448). Lives in a separate bundle: SharedWorker
+// scripts run in a different global (`self` is the worker scope, no
+// `window`) so they can't be inlined into app.js / flow.js. Output is
+// at `static/stream-worker.js`; common.js's `openStream` references
+// `/static/stream-worker.js` as the SharedWorker URL. ES-module-shaped
+// (so a future `import` from within the worker can pull in shared
+// utilities), browser target — webworker target subset.
+await build({
+  entryPoints: [src('stream-worker.js')],
+  outdir: staticDir(''),
+  bundle: true,
+  format: 'esm',
+  platform: 'browser',
+  target: ['es2022'],
+  sourcemap: true,
+  logLevel: 'info',
+});
+
 // Bundle the CSS — esbuild resolves @import including the package
 // re-exports from @hive/shared.
 await build({