Add loading skeleton for sidebar

[joshdayorg]

Summary

Fixes #23

The Problem

When loading the app, the sidebar shows blank white space for 3+ seconds while content loads. This makes the page look broken, as described in the issue.

The Solution

Add a loading skeleton that appears immediately while the sidebar content loads asynchronously. The skeleton mimics the sidebar structure:

• Direct messages row (5 avatar placeholders)
• Search input placeholder
• "My Rooms" section (heading + 4 room placeholders)
• "All Rooms" section (heading + 6 room placeholders)

Implementation

Uses the same pattern as accounts/users/_next_page_container.html.erb:

1. When sidebar_turbo_frame_tag is called with src: and no block, render the skeleton
2. Turbo automatically replaces skeleton with real content when loaded

Changes

• app/views/users/sidebars/_loading_skeleton.html.erb - New skeleton partial
• app/helpers/users/sidebar_helper.rb - Render skeleton when src provided
• app/assets/stylesheets/application/sidebar.css - Skeleton styles with pulse animation

Browser Support

Uses CSS variables already defined in the codebase. Animation works in all modern browsers.

Testing

• Full page refresh shows skeleton immediately
• Skeleton replaced by real sidebar when loaded
• No layout shift between skeleton and content
• Works on mobile and desktop