feat: add structured error types for Tauri IPC commands

Introduces McError and McErrorCode to replace string-based errors in
Tauri commands. This enables the frontend to handle errors programmatically
rather than parsing error messages.

Key design decisions:
- Error codes use SCREAMING_SNAKE_CASE for frontend pattern matching
- Each error indicates whether it's recoverable (user can retry)
- Automatic conversion from LoreError, BeadsError, and BridgeError
- Errors serialize to JSON for consistent IPC transport

Error categories:
- Lore: LORE_UNAVAILABLE, LORE_UNHEALTHY, LORE_FETCH_FAILED
- Bridge: BRIDGE_LOCKED, BRIDGE_MAP_CORRUPTED, BRIDGE_SYNC_FAILED
- Beads: BEADS_UNAVAILABLE, BEADS_CREATE_FAILED, BEADS_CLOSE_FAILED
- General: IO_ERROR, INTERNAL_ERROR

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src-tauri/src/data/mod.rs

@@ -5,6 +5,7 @@
 //! - br CLI (beads task management)
 //! - MC local state (mapping, decisions, settings)
 
-pub mod lore;
 pub mod beads;
+pub mod bridge;
+pub mod lore;
 pub mod state;