fix: preserve Settings Editor TOC section state across focus changes (fixes PlatformNetwork/bounty-challenge#21908)

[echobt]

Fix

Preserves the Settings Editor TOC (Table of Contents) section state across editor focus changes.

Changes

• SettingsEditor.tsx: Module-level signal persistedActiveSection is now backed by sessionStorage via safeGetItem/safeSetItem
• The active section is only updated when the user explicitly clicks a TOC section, not reactively overwritten on focus changes
• SessionStorage key settings_active_toc_section ensures persistence across component unmount/remount cycles

Root Cause

When the editor gained/lost focus, the SettingsEditor component could re-mount (due to <Show>/<Switch> wrappers), and the module-level signal would be reset to 'editor' (default). The fix backs the signal with sessionStorage so it survives these cycles.

Fixes PlatformNetwork/bounty-challenge#21908