fix: Make many concurrent ompweb tabs work

[abread]

Problem

Browsers allow only six HTTP/1.1 connections per host, shared by every tab, and each long-lived SSE response holds one open for as long as it lives. Each omp-web tab opened up to four:

Stream	Opened by
/api/agent/running/events?clientId=	sidebar (running badges, session-list hints, the tab's notifications)
/api/agent/host-tools/events	AppShell (cross-session open_url / notify / open_file)
/api/agent/[id]/events	chat (agent events for the open session)
/api/files/...?type=watch	file viewer (changes to the open file)

Two or three working tabs used up the limit. After that, every other request from every tab (sending a message, loading a session, reading a file) waited in the browser's queue until a tab was closed. The app looked frozen.

Fix

Use only one connection per browser, carrying many subscriptions:

flowchart LR
    T1[Tab 1] --> W
    T2[Tab 2] --> W
    T3[Tab N] --> W
    W[SharedWorker] -->|"one SSE: GET /api/events"| S[Server hub]
    W -->|"POST /api/events: subscribe/unsubscribe"| S
    S --> C1[running]
    S --> C2[notifications]
    S --> C3[host-tools]
    S --> C4[session]
    S --> C5[file]

Loading

• /api/events (lib/event-stream-hub.ts, lib/event-stream-protocol.ts): a single SSE response that carries any number of channel subscriptions, added and removed with POST /api/events. Each subscription's frames arrive as event: <subId>;
  connection control frames arrive as event: mux (hello / open / end).
• public/event-stream-worker.js: a SharedWorker that holds the browser's only /api/events EventSource and multiplexes every tab's subscriptions over it. Where SharedWorker is missing (e.g. Chrome for Android) it runs as a dedicated Worker, so
  each tab still uses a single connection. Closed tabs are detected with a Web Lock held for the tab's lifetime. The worker URL includes a content hash, so a new version never serves tabs still running the old one.
• lib/event-stream.ts: openEventStream(channel) returns an object that behaves like an EventSource (onerror while CONNECTING means it's reconnecting on its own; CLOSED means it gave up). The sidebar, shell, chat hook, file viewer and
  notifications hook only needed their constructor swapped.

Channels are split by who subscribes, how long it lives, what it's keyed on and which access check applies:

Channel	Carries	Subscribed from
running	running-session ids, session-list refresh hints	sidebar
notifications	toasts and in-page system notifications for one tab (clientId)	useNotifications in AppShell
host-tools	cross-session host tool calls	AppShell
session	agent events for one live session (watch-only: it never starts an omp process)	chat hook
file	changes to one file (same access check as /api/files)	file viewer

Comparison with behaviour from the old streams:

• While the connection is backed up, message_update frames for a session are still merged so only the latest is sent.
• A frame that can't be serialized now ends only its own subscription. Before, it would close the shared connection that every other tab depends on.
• File-access checks are pulled into lib/file-authorization.ts: resolving ~/ and workspace-relative paths, and refusing omp's credential files under the config folder. /api/files and the file channel now run the same check.
• The provider-login stream (ModelsConfig) and the clone progress stream (DirectoryPicker) still open their own connections. Each lasts only for one user action, so they don't pile up across tabs.
• Removed: /api/agent/[id]/events, /api/agent/running/events, /api/agent/host-tools/events, /api/files?type=watch, and the window event the sidebar used to relay notifications to useNotifications.

User-visible changes

• Any number of tabs keeps working; requests no longer pile up behind open streams.
• A tab on Settings now gets notifications. They used to ride the sidebar's stream, and the sidebar unmounts while Settings is open. So that tab got no toasts and counted as unreachable, and its notifications went to your other devices instead. They
  now have their own channel, mounted in the shell.

Testing

• npm test: 1409 pass, 0 fail, at the tip and at commit 2 on its own. Typecheck is clean; lint has no new warnings.
• New and moved tests cover each channel: subscribe/unsubscribe, listener leaks on disconnect, the file access check, merged updates under backpressure, notification reachability and client-id validation, and host-tool delivery. The worker suite covers
  sharing between tabs, reconnects, server restarts, and closed tabs.
• Manual check in headless Chromium with 2–3 tabs: one shared worker held one open /api/events connection carrying every tab's subscriptions, and the pages themselves opened no EventSources. Switching a tab to Settings dropped its running
  subscription but kept notifications and host-tools.