feat: add experimental workspace sidebar persistence

[AnrokX]

Summary

• add an off-by-default Experimental setting to persist workspace sidebar selections and filters
• restore per-workspace hidden worktrees plus View/Tier filters across tabs, reloads, and app/server restarts
• cover the new user settings defaults/merge behavior and document the experimental setting

Test plan

• node --check server/index.js
• node --check server/userSettingsService.js
• node --check client/app.js
• node --check client/workspace-tab-manager.js
• npx jest tests/unit/userSettingsDefaults.test.js