feat(desktop): open any chat in its own window (#43219)

Pops a session into a standalone, focused window for side-by-side work.
A secondary window loads the renderer at the session route with a
?win=secondary flag (ahead of the HashRouter '#'); it drops the global
sidebar plus the install/onboarding overlays and renders a single chat,
sharing the one local gateway over WS (no backend duplication). The main
process keys windows by sessionId so re-opening focuses the existing one
and self-cleans on close.

Open it via:
- ⌘-click (mac) / ⌃-click (win/linux) a sidebar session — the universal
  "open in new window" gesture. Archive moves to the ⋯ / right-click menus
  only, off the easy-to-misfire modifier-click.
- "New window" in the session ⋯ and context menus (link-external icon,
  i18n'd across en/ja/zh/zh-hant).

A standalone window has no left rail, so AppShell treats its edge as
uncovered and applies the titlebar inset — the chat title clears the
macOS traffic lights instead of hiding behind them.

Co-authored-by: tim404x <tim404x@users.noreply.github.com>

apps/desktop/src/global.d.ts

@@ -18,6 +18,10 @@ declare global {
       // reaper spares it while its chat is active.
       touchBackend: (profile?: string | null) => Promise<{ ok: boolean }>
       getGatewayWsUrl: (profile?: null | string) => Promise<string>
+      // Open (or focus) a standalone OS window for a single chat session so
+      // the user can work with multiple chats side by side. Returns ok:false
+      // with an error code when the sessionId is empty/invalid.
+      openSessionWindow: (sessionId: string) => Promise<{ ok: boolean; error?: string }>
       getBootProgress: () => Promise<DesktopBootProgress>
       getConnectionConfig: (profile?: null | string) => Promise<DesktopConnectionConfig>
       saveConnectionConfig: (payload: DesktopConnectionConfigInput) => Promise<DesktopConnectionConfig>