SB 9.1 Create BrainSwitcher component

[carlgombert]

Task 9.1: Create BrainSwitcher component

Wave: 11

Description

• Create crates/memento-app/frontend/src/components/layout/BrainSwitcher.tsx
• Display active brain name (truncated to 17 chars + "..." if > 20 chars)
• Implement dropdown with brain list (scrollable, max 50 visible)
• Add inline "Create new Brain" option with name input and validation
• Add context menu per brain entry with "Rename" and "Delete" options
• Show confirmation dialog before deletion
• Show loading indicator during brain switch (isSwitching state)
• Disable brain selection controls while switching

Requirements

• 8.1, 8.2, 8.3, 8.4, 8.5, 8.6, 8.7, 8.8