feat(desktop): server tabs, several Codeman servers open in one window

The Mac app showed one server at a time, and moving to another machine
meant Switch Server and a fresh connect. Now every server gets a tab in
the title bar, and every tab stays live.

- Tab strip: a small bundled page (assets/tabs.html) in its own web view
  across the top of the window, with the traffic lights inset into it.
  Each tab has a colour and a status badge: a red count when sessions
  need you, an amber dot when they are waiting, a blue ring while they
  work, a dashed circle while the server does not answer.
- The status comes from the app, not from the page: one thread per tab
  reads GET /api/sessions every 2 seconds (macOS stalls hidden web
  views), backs off for 60 s on a 429 and stops on 401/403, so a wrong
  password is never retried in a loop.
- + or ⌘T opens the connect screen in a new tab. A server that is
  already open just brings its tab forward instead of opening twice,
  also when two connects race (checked again when the probe ends).
- Double-click or the right-click menu renames a tab; the same menu
  sets the colour, reloads, reconnects, closes, or closes the others.
  Tabs can be dragged. A new Server menu carries ⌘T, ⌘W, ⇧⌘T (reopen
  closed tab, last 10), ⌘0, ⇧⌘[ / ⇧⌘] and ⌘1 to ⌘8 (⌘9 is the last
  tab once there are nine). Close Window moves to ⇧⌘W. Ctrl+Tab and
  Alt+1..9 stay with Codeman's own session switching.
- ⌘0 shows every server at once (assets/overview.html): the ones that
  need you first, up to four sessions each with what they ask, and a
  click on a session opens its tab and selects it.
- Tabs are saved in desktop.json (tabs, activeTab) and come back at
  launch, connecting by themselves when auto-connect is on.
- Logins: each tab keeps its own Basic credentials, and a challenge is
  answered only with the login of the tab whose web view asked
  (macos.rs binds each WKWebView to its tab).
- Cookies: on macOS 14 and later every server gets its own WebKit data
  store (WKWebsiteDataStore dataStoreForIdentifier, with a stable id
  hashed from the server URL), so a server on port 3000 and a beta on
  5000 of the same host cannot log each other out. Before macOS 14 a
  second tab on a host that is already open is refused with a message.
  The server used last before this change keeps the default store, so
  its per-device settings (font size, skin) survive; every other
  server starts those fresh once.
- The lid logo blink watches all open servers, and Keep This Mac Awake
  follows the tab that runs on this Mac.

New pure model in src/tabs.rs (names, colours, order, shortcuts, strip
and overview state) with unit tests; agents.rs reads the per-session
detail for the badges and the overview; policy.rs parses the strip and
overview IPC and refuses it from any other frame. Browser tests drive
both bundled pages (test/desktop-tabs-pages.browser.test.ts) and the
connect page's restored-tab path.

Not yet run in a real window: the Mac mini has no screen session. The
owner checks the traffic-light placement, clicks under the transparent
title bar, window drag and zoom from the strip, keyboard focus after
strip clicks, and two logged-in servers side by side.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.changeset/desktop-server-tabs.md

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+"aicodeman": patch
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+**Mac app: server tabs.** The Mac app now keeps several Codeman servers open in one window, one tab each in the title bar, and switches between them with a click or ⌘1 to ⌘8. Every tab shows whether its server needs you, is waiting, is working or is offline, and ⌘0 shows all servers at once with the ones that need you first. ⌘T opens another server, a tab can be renamed, coloured and dragged, ⇧⌘T reopens a closed one, and your tabs come back at the next launch. On macOS 14 and later every server keeps its own login and cookies; before macOS 14 a second tab on the same machine is refused. Behaviour change: only the server you used last keeps its per-device settings (font size, skin); every other server starts those fresh once.