fix: keep client router.replace relative for sub-path deployments

[c-with]

What does this PR do?

Three router.replace("/") calls in AppShell.tsx (handleCwdChange, handleNewSession, branch-switch cleanup) push the browser to the origin root, discarding the mount prefix on deployments served behind a path-prefix reverse proxy (e.g. example.com/pi/). The user lands on a 404/blank root page after switching project, creating a session, or switching branch.

Fix: replace "/" with window.location.pathname so the replace keeps the current mount prefix while still clearing the ?session= query param. The original intent (drop the session query) is preserved — only the destination changes.

Related Issue

No existing issue found for this. (Happy to file one first if preferred — sub-path deployments were discussed in #370 / #50 but those were about basePath support, which stays out of scope here.)

• 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

components/AppShell.tsx — 3 call sites, "/" → typeof window !== "undefined" ? window.location.pathname : "/".

Verification

Deployed behind an Nginx reverse proxy at a /pi/ prefix (proxy strips the prefix, no basePath — same shape as #370 discussion): before the fix, project switch / new session / branch switch all navigate the browser to the origin root; after, they stay on the mounted prefix and only the ?session= param clears. Also ran components/AppShell* vitest suites (pass) and tsc --noEmit (clean).

中文摘要：子路径反代部署下 router.replace("/") 会把浏览器顶到域名根路径，切换项目/新建会话/切换分支后直接 404。改为 window.location.pathname 保留挂载前缀，仅清除 session 查询参数——原语义不变，只改目的地。