fix(content): hide empty settings panel

[DavidBabinec]

Summary

• Gate the shared workspace right sidebar on real right-panel content so persisted open state cannot reserve an empty rail.
• Preserve a closed Content settings preference when selection is cleared.
• Add Content regressions for empty installs and deleting the last selected entry.

Root Cause

The shared workspace layout expanded from persisted right-panel state without checking whether the current workspace had right-panel content. Content also reopened the right panel for selectEntry(null), so clearing the active entry could rewrite the preference to open.

Impact

Fresh installs and empty Content collections now show the full canvas without an empty right rail. Selecting or creating an entry still opens the settings panel, and the reopen notch still appears only when an entry-backed panel exists.

Verification

• bun run build
• bun test
• bun run lint
• bunx react-doctor@latest --verbose --diff
• bunx react-doctor@latest --verbose --scope changed --base main