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/app/shell/app-shell.tsx

@@ -16,6 +16,7 @@ import {
 } from '@/store/layout'
 import { $paneWidthOverride } from '@/store/panes'
 import { $connection } from '@/store/session'
+import { isSecondaryWindow } from '@/store/windows'
 
 import { SIDEBAR_COLLAPSE_MEDIA_QUERY } from '../layout-constants'
 
@@ -77,8 +78,10 @@ export function AppShell({
   // window's left edge. Default layout: the sessions sidebar sits there.
   // Flipped layout: the file browser does instead. Below the collapse
   // breakpoint both rails are force-collapsed (hover-reveal overlay), so the
-  // edge is uncovered regardless of their stored open state.
-  const leftEdgePaneOpen = !narrowViewport && (panesFlipped ? fileBrowserOpen : sidebarOpen)
+  // edge is uncovered regardless of their stored open state. A standalone
+  // session window renders no sidebar at all, so its edge is always uncovered.
+  const leftEdgePaneOpen =
+    !narrowViewport && !isSecondaryWindow() && (panesFlipped ? fileBrowserOpen : sidebarOpen)
 
   const titlebarContentInset = leftEdgePaneOpen
     ? 0