fix: keep desktop sidebar independently scrollable

[bonheur15]

Summary

• keep the desktop sidebar pinned to the viewport while the page scrolls
• constrain the navigation region so long navigation scrolls independently
• keep the sidebar header and footer fixed and reachable
• preserve the fixed mobile drawer behavior

Type of change

• Bug fix

Areas touched

• apps/web (React / Vite)

Related issues

Closes #121

How to test

1. Open a long dashboard page at a desktop breakpoint and scroll the page; the sidebar should remain in place.
2. Reduce the viewport height until the navigation overflows; only the navigation region should scroll and the footer should remain visible.
3. Switch to a mobile breakpoint, open and close the navigation drawer, and confirm its navigation remains scrollable.

Checklist

• Change is focused and stays within the existing modular monolith boundaries.
• Ran the relevant checks locally.
• No documentation, protocol, or module-boundary changes are required.

Local checks

• npm run lint --workspace=@docksight/web (passes with three existing warnings in unrelated files)
• npm run test --workspace=@docksight/web (44 tests passed)
• npm run build --workspace=@docksight/web
• git diff --check

Screenshots

Desktop — constrained viewport

The navigation region has its own scrollbar while the account, theme, and version footer remains visible.

Mobile drawer

The existing mobile drawer remains full-height and keeps its footer controls reachable.