fix(explorer): budget Squid deposits with live fees

Parent tracker: #436
Owning PR: #421
Depends on: #399 / PR #420

## Goal

Price every planned Squid source transaction with current network fees and bounded headroom before the user signs.

## Scope

- Estimate fees for every transaction in the planned route.
- Preflight the whole plan, including approvals, before signing begins.
- Refresh the route when the reviewed cost becomes stale.
- Preserve the bounded OP Stack fallback when estimation is unavailable.

## Non-goals

- Redesigning the Squid funding flow.
- Changing token or route selection policy.
- Restacking, merging, or otherwise changing PR state.

## Acceptance criteria

- The Ethereum USDT approval path cannot spend more native gas than the reviewed maximum.
- A fee or balance failure stops before signing, or requires an explicit re-review.
- Fallback headroom is bounded and visible to the existing review flow.
- Focused tests cover estimation, approval transactions, stale quotes, and failure paths.

## Performance

Performance is relevant because fee polling and simulations run while the dialog is open. Requests must remain bounded and must not materially reduce dialog responsiveness.

## Delivery

PR #421 remains based on PR #420 while the stack is open. After its predecessor merges, retarget it to staging and revalidate the exact head.

Start with the existing Squid funding flow, approval path, and review dialog, then trace how the planned route is estimated before signing. Focused tests should cover estimation, approval transactions, stale quotes, and failure paths. Done means the acceptance criteria pass without materially reducing dialog responsiveness; PR #421 and its predecessor must also be accounted for.

Written by the indexing model from the issue text.