refactor: extract shared layout components from default and paper themes

[rohilsurana]

Problem

The default and paper themes duplicate significant layout logic:

• Sidebar rendering and tree traversal
• Version switcher component
• Content directory buttons/dropdown
• Mobile menu handling
• Icon rendering from config

This makes bug fixes and feature additions (like new sidebar features) require changes in two places.

Suggested Fix

Extract shared components into `src/themes/shared/` or `src/components/layout/`:

• `SidebarTree` — recursive tree rendering with configurable depth, icons, and active state
• `VersionSwitcher` — already exists as separate files but with duplicated logic
• `ContentDirNav` — content directory buttons/dropdown selector
• `MobileMenu` — slide-out menu with sidebar content

Each theme would compose these shared components with its own layout structure and styling.