refactor(backend): phase 1 - unified error handling with thiserror
Introduce AppError enum to replace Result<T, String> pattern across
the codebase, improving error context preservation and type safety.
## Changes
### Core Infrastructure
- Add src/error.rs with AppError enum using thiserror
- Add thiserror dependency to Cargo.toml
- Implement helper functions: io(), json(), toml() for ergonomic error creation
- Implement From<PoisonError> for automatic lock error conversion
- Implement From<AppError> for String to maintain Tauri command compatibility
### Module Migrations (60% complete)
- config.rs: Full migration to AppError
- read_json_file, write_json_file, atomic_write
- archive_file, copy_file, delete_file
- claude_mcp.rs: Full migration to AppError
- get_mcp_status, read_mcp_json, upsert_mcp_server
- delete_mcp_server, validate_command_in_path
- set_mcp_servers_map
- codex_config.rs: Full migration to AppError
- write_codex_live_atomic with rollback support
- read_and_validate_codex_config_text
- validate_config_toml
- app_config.rs: Partial migration
- MultiAppConfig::load, MultiAppConfig::save
- store.rs: Partial migration
- AppState::save now returns Result<(), AppError>
- commands.rs: Minimal changes
- Use .map_err(Into::into) for compatibility
- mcp.rs: Minimal changes
- sync_enabled_to_claude uses Into::into conversion
### Documentation
- Add docs/BACKEND_REFACTOR_PLAN.md with detailed refactoring roadmap
## Benefits
- Type-safe error handling with preserved error chains
- Better error messages with file paths and context
- Reduced boilerplate code (118 Result<T, String> instances to migrate)
- Automatic error conversion for seamless integration
## Testing
- All existing tests pass (4/4)
- Compilation successful with no warnings
- Build time: 0.61s (no performance regression)
## Remaining Work
- claude_plugin.rs (7 functions)
- migration.rs, import_export.rs
- Add unit tests for error.rs
- Complete commands.rs migration after dependent modules
Co-authored-by: Claude <claude@anthropic.com>
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