refactor(mcp): preserve TOML formatting when syncing to Codex

Switch from `toml` to `toml_edit` crate for incremental TOML editing,
preserving user-written comments, whitespace, and key ordering in
Codex config.toml.

Changes:
- Add toml_edit 0.22 dependency for preserving-style TOML editing
- Refactor sync_enabled_to_codex() to use toml_edit::DocumentMut API
- Implement smart style detection: inherit existing mcp.servers or
  mcp_servers style, with fallback to sensible defaults
- Add deduplication logic to prevent both styles coexisting
- Add tests for format preservation and style inheritance
- Fix unused_mut and nonminimal_bool compiler warnings
- Apply cargo fmt to all modified files

Benefits:
- User comments and formatting are no longer lost during sync
- Respects user's preferred TOML structure (nested vs toplevel)
- Non-MCP fields in config.toml remain untouched
- Minimal surprise principle: only modifies necessary sections

Testing:
- All 47 tests passing (unit + integration)
- Clippy clean (0 warnings, 0 errors)
- Release build successful
- New tests verify comment preservation and style detection

src-tauri/Cargo.toml

@@ -35,6 +35,7 @@ tauri-plugin-dialog = "2"
 tauri-plugin-store = "2"
 dirs = "5.0"
 toml = "0.8"
+toml_edit = "0.22"
 reqwest = { version = "0.12", default-features = false, features = ["rustls-tls", "json"] }
 tokio = { version = "1", features = ["macros", "rt-multi-thread", "time"] }
 futures = "0.3"