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/i18n/en.ts

@@ -1084,6 +1084,7 @@ export const en: Translations = {
       export: 'Export',
       rename: 'Rename',
       archive: 'Archive',
+      newWindow: 'New window',
       copyIdFailed: 'Could not copy session ID',
       actionsFor: title => `Actions for ${title}`,
       sessionActions: 'Session actions',

apps/desktop/src/i18n/ja.ts

@@ -1218,6 +1218,7 @@ export const ja = defineLocale({
       export: 'エクスポート',
       rename: '名前を変更',
       archive: 'アーカイブ',
+      newWindow: '新しいウィンドウ',
       copyIdFailed: 'セッション ID をコピーできませんでした',
       actionsFor: title => `${title} のアクション`,
       sessionActions: 'セッションアクション',

apps/desktop/src/i18n/types.ts

@@ -832,6 +832,7 @@ export interface Translations {
       export: string
       rename: string
       archive: string
+      newWindow: string
       copyIdFailed: string
       actionsFor: (title: string) => string
       sessionActions: string

apps/desktop/src/i18n/zh-hant.ts

@@ -1184,6 +1184,7 @@ export const zhHant = defineLocale({
       export: '匯出',
       rename: '重新命名',
       archive: '封存',
+      newWindow: '新視窗',
       copyIdFailed: '無法複製工作階段 ID',
       actionsFor: title => `${title} 的動作`,
       sessionActions: '工作階段動作',

apps/desktop/src/i18n/zh.ts

@@ -1271,6 +1271,7 @@ export const zh: Translations = {
       export: '导出',
       rename: '重命名',
       archive: '归档',
+      newWindow: '新窗口',
       copyIdFailed: '无法复制会话 ID',
       actionsFor: title => `${title} 的操作`,
       sessionActions: '会话操作',