Web settings: slippage and preferences store

[0xMegie]

Area: apps/web/src/features/settings/store/preferences-store.ts

Goal: Lock down slippage tolerance and user preference transitions.

Tasks:

• Test default slippage, custom value, and invalid/out-of-range input.
• Test persistence and rehydration from storage.
• Keep tests pure and deterministic.

Acceptance:

• Store state and storage do not leak between tests.
• bun run --cwd apps/web test passes.

[pablowizzy24]

Stores user slippage tolerance and app preferences, providing a consistent and personalized experience across sessions.