opt.: use GoRouter and multi-navigator for desktop

[lollipopkit]

• Use go_router with StatefulShellRoute.indexedStack to build a desktop Shell (NavigationRail + content area).
• Create one Navigator per sidebar destination (branch navigator keys) to keep independent stacks per section.
• Keep a Root Navigator for global routes/overlays (login, full-screen flows, app-wide dialogs via rootNavigator).
• Define deep-linkable paths for all detail pages (e.g., /inbox/thread/:id).
• Back/shortcuts: pop current branch first (PopScope + maybePop); support Esc / Cmd+[ / Alt+Left.
• Responsive: same routing; only swap UI (Rail on desktop, BottomNav/Drawer on narrow screens).

[GT-610]

我觉得方向是对的，尤其是 desktop 这边，但我看了下现在项目的导航结构，真要按 go_router + StatefulShellRoute.indexedStack 做，基本已经不是单纯桌面端优化了，而是全平台路由重构，移动端也会一起被带动。所以我倾向于先把它记成“导航架构升级方向”，后面结合实际痛点再拆分推进，而不是现在直接整体切过去。这样风险会更可控一些。