Improve rooms sidebar load performance with lazy loading

[kyleorin]

Reduces sidebar load time by deferring involvement button rendering until user interaction.
Changes

Involvement buttons now load on hover/focus instead of initial page load
Static placeholder icons display immediately, replaced by interactive buttons when needed
Added system test for lazy loading behavior

How it works

Each room item shows a lightweight placeholder icon. When users hover over a room, a Turbo Frame fetches the actual involvement button. This reduces initial DOM size significantly.
Files changed

app/views/users/sidebars/rooms/_shared.html.erb - Lazy loading via Turbo Frames
app/frontend/controllers/lazy_involvement_controller.js - Stimulus controller for on-demand loading
app/frontend/controllers/sidebar_loading_controller.js - Loading state management
app/assets/stylesheets/application/sidebar.css - Placeholder and loading styles
test/system/sidebar_lazy_loading_test.rb - System test

Closes #23
AI Disclosure

Model: Claude Opus 4.5 via Claude Code
Used for: Codebase exploration, understanding existing patterns, code generation (Stimulus controllers, CSS, ERB templates), test writing
All code reviewed and follows existing codebase patterns

Fixes #23