fix(desktop): restore sidebar sessions with projects

[yahya3867]

Summary

• Restore the flat Sessions roster as its own section when the project-grouped sidebar is active.
• Keep the Projects browser scoped to project overview/project content instead of replacing recents.
• Add a regression test for the visibility rule so project overview keeps flat sessions available.

Fixes #57968.

Tests

• npm run test:ui --workspace apps/desktop -- sidebar/visibility.test.ts
• npm run test:ui --workspace apps/desktop -- sidebar
• npm run build --workspace apps/desktop

Notes

• Checked issue bug(desktop): flat "Sessions" list missing from sidebar after update (only Pinned/Projects show, search still works) #57968 immediately before pushing: open, no assignee, no linked PR.
• Related open PR fix(desktop): show unprojected sessions in project grouping #54608 handles unprojected session membership inside the project tree; this PR restores the separate flat Sessions section.

[teknium1]

Thanks for the focused desktop sidebar fix. The premise remains valid on current main: ChatSidebar still makes its only section a Projects/Sessions switcher (apps/desktop/src/app/chat/sidebar/index.tsx:752-756,1318-1329), while SidebarSessionsSection renders projectOverview before its flat sessions input (apps/desktop/src/app/chat/sidebar/sessions-section.tsx:238-334).

The PR addresses that mechanism directly by retaining the grouped project browser with an empty flat input and rendering displayAgentSessions in a separate Sessions section (apps/desktop/src/app/chat/sidebar/index.tsx:1334-1401 at f32921681ab3). The visibility helper tests the three relevant grouped/project-scope states. GitHub currently reports the PR as mergeable.

Automated hermes-sweeper review.