feat: user card popup on username click

[ImpulseB23]

Clicking a username in the chat feed should open a user card popup (similar to Chatterino). The card should show:

• Display name and username
• Profile picture (requires Helix Get Users call, can be cached)
• Follow status / account age
• Mod actions (timeout, ban) if the viewer has mod privileges

This requires:

• A new Tauri command to fetch user profile data via the sidecar (Helix GET /users)
• Client-side caching of user profiles (LRU, keyed by user_id)
• A popup/card component positioned near the clicked username
• Mod action buttons that invoke existing or new sidecar commands

The platform_user_id is already available on every ChatMessage, so the lookup key is ready.