SB 5.2 Implement Tauri brain commands

[carlgombert]

Task 5.2: Implement Tauri brain commands

Wave: 9

Description

• Create crates/memento-app/src/commands/brain.rs with commands: create_brain, list_brains, switch_brain, rename_brain, delete_brain, get_active_brain
• Register commands in crates/memento-app/src/commands/mod.rs and the Tauri builder in main.rs
• Map BrainError variants to user-friendly error strings

Requirements

• 1.1, 2.1, 3.1, 4.1, 5.1, 5.6