Extract RaindexOrderBuilder from js_api into common crate

[findolor]

Summary

crates/js_api/src/gui/ (~7,850 lines across 7 files) contains the order-building state machine, validation, and transaction generation logic. ~90% is platform-agnostic business logic trapped in the WASM-only crate. Rename from gui to raindex_order_builder to reflect its actual purpose.

What needs to move

Current file	New module	Lines	Core %	Description
gui/mod.rs	raindex_order_builder	2,120	~85%	RaindexOrderBuilder state machine, config, deployment management
gui/order_operations.rs	raindex_order_builder::calldata	1,215	~95%	Transaction calldata (approvals, deposits, addOrder, multicall)
gui/field_values.rs	raindex_order_builder::fields	1,008	~90%	Field value parsing, presets, get/set/unset
gui/select_tokens.rs	raindex_order_builder::tokens	1,036	~85%	Token selection, RPC metadata fetching
gui/state_management.rs	raindex_order_builder::state	950	~100%	Serialize/restore state (gzip+base64+SHA256)
gui/deposits.rs	raindex_order_builder::deposits	710	~95%	Deposit state tracking, preset detection
gui/validation.rs	raindex_order_builder::validation	703	~100%	Number/string/boolean constraint checking

Key renames

• DotrainOrderGui → RaindexOrderBuilder
• GuiError → RaindexOrderBuilderError
• gui/ → raindex_order_builder/

What stays in js_api

Thin WASM wrappers for each module:

• #[wasm_bindgen] on a WASM-facing struct
• #[wasm_export] on all public methods
• js_sys::Function state update callback binding
• Error-to-JsValue conversions

Proposed approach

1. Create common::raindex_order_builder module with core RaindexOrderBuilder struct
2. Replace js_sys::Function callback with generic Rust callback trait/closure
3. Keep js_api::gui (or rename to js_api::order_builder) as thin WASM wrappers
4. Ensure RaindexOrderBuilder is usable from CLI and tests without WASM

Benefits

• CLI can use the same order-building/validation logic
• Tests can verify state machine without WASM target
• Server-side tooling can reuse field validation and calldata generation
• Clear separation: core logic vs platform bindings
• Name reflects actual purpose (builder, not GUI)