feat(app): simplify experimental settings - #49373

Summary

• move the tab layout preference directly below language in Preferences
• enable the desktop browser pane whenever the platform supports it and retire the project-name and project-icon experiments
• hide the empty Experimental page behind one availability flag so it can return when new settings are added
• remove retired settings from persistence and settings search

Testing

• bun typecheck (packages/app)
• bun test src/settings/model.test.ts src/settings/search-results.test.ts (packages/app)
• bun typecheck (packages/plugin-browser)
• focused Playwright coverage for tab layout, hidden Experimental navigation, desktop settings padding, and mobile settings padding
• real-app visual validation at 1280×800 in dark mode

Requested by: @iamdavidhill (David via Slack)