fix(ui): keep sidebar scroll when switching emails

[ivenuss]

Issue

Navigating between emails remounted Shell on every page, so the sidebar lost its scroll position.

Fix

Move Shell into the root layout so it stays mounted across routes, and derive the active email from useParams() instead of passing currentEmailOpenSlug down the tree.

Demonstration of the issue

ui.mov

---

Summary by cubic

Preserves the sidebar scroll position when switching between emails by keeping the shell mounted and deriving the active email from the route.

• Bug Fixes
  • Moved Shell to app/layout.tsx so the sidebar stays mounted across routes.
  • Removed per-page Shell wrappers in app/page.tsx and app/preview/[...slug]/page.tsx.
  • Sidebar reads the open email slug from useParams() (decodes joined slug); removed the currentEmailOpenSlug prop.
  • Small cleanup in shell.tsx and sidebar.tsx.

^{Written for commit 9538435. Summary will update on new commits.}

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Latest commit: 9538435

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