feat(app): add title bar with conversation actions

[comppaz]

Summary

Add a title bar to the chat interface that displays the current conversation title with pin and delete action buttons.

Changes

• Title bar appears when a conversation is selected
• Shows conversation title with document icon
• Pin/unpin button to favorite conversations
• Delete button with confirmation dialog
• Sidebar delete also shows confirmation dialog
• Sidebar icons now overlay with gradient fade (no reserved space)

Related

Extracted from PR #37