feat(widget): add limit order mode

[effie-ms]

Which Linear task is linked to this PR?

https://linear.app/lifi-linear/issue/EMB-322

Why was it implemented this way?

Adds a limit order mode as a dedicated header tab. The core challenge is keeping three interdependent values — send amount, receive amount, and limit price — coherent without circular updates:

• Linked fields (useLinkedLimitFields) implement one-directional derivation per edit: editing send recomputes receive (from price), editing receive recomputes price, editing price recomputes receive. Send never changes without an explicit user edit.
• Canonical price storage — the limit price is stored as receive-per-send and only flipped at the display layer (LimitPriceCard), so the inversion toggle, market-proximity indicator, and +N% presets all reason about a single source of truth regardless of which direction is shown.
• State reset on pair change — switching tokens or re-entering limit mode clears the stale price/receive so a price from a previous pair can't drive a wrong amount or a false off-market warning.
• Route fetching is wired but gated — the installed @lifi/sdk@4.0.0 can carry neither the extra limit fields (toAmount/validUntil/partiallyFillable) nor a per-call base URL, so a dedicated client (services/limitOrder) mirrors advanced/routes. It is intentionally inactive pending the limit-order API reaching a stable environment — see TODO(EMB-323) in useRoutes. Those stub files are added to knip.json's ignore list until they're activated.

Visual showcase (Screenshots or Videos)

TODO: attach a screen recording of the limit price card, presets, invert toggle, and expiry/partial-fill settings.

Checklist before requesting a review

• I have performed a self-review and testing of my code.
• This pull request is focused and addresses a single problem.
• If this PR modifies the Widget API or adds new features that require documentation, I have updated the documentation in the public-docs repository.

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