feat(explorer): add balance-aware Squid payment sources

> [!IMPORTANT]
> Current execution is tracked by #436. This issue now owns the curated, balance-aware, bounded direct-deposit source inventory in PR #416, after #395 / PR #415. It does not replace the broader historical work tracked in #343 or #356. Historical references below are retained for attribution.

Parent tracker: #393
Depends on: #395
Supersedes after landing: #343, #344, and #356 for the direct-funding path

## Goal

Expand direct Squid deposits from USDC-only to supported wallet-held assets while keeping selection, balances, quote totals, and execution inputs authoritative.

## Scope

- Add the curated supported-chain and supported-token catalog.
- Read native and ERC-20 balances in bounded, failure-tolerant batches.
- Show funded pairs first with USDC first, while preserving access to the full supported catalog.
- Use the selected chain/token pair as the sole owner of displayed balance and quote state.
- Refresh balance, native gas, allowance, and route inputs before execution.
- Present source spend, native gas, Squid fees, minimum received, and maximum requirement clearly.

## Acceptance criteria

- [ ] ETH, USDT, DAI, WETH, WBTC, and USDC work on the explicitly supported networks.
- [ ] Duplicate symbols are disambiguated.
- [ ] Selection remains stable while inventory refreshes; results do not reorder under the pointer.
- [ ] Failed balance calls remain unknown rather than becoming false zeroes.
- [ ] Account/network changes cannot display or execute stale state.
- [ ] Unit and component tests cover batching, partial failure, selection ownership, quote ownership, and pre-execution refresh.
- [ ] Repository-required checks pass.

Performance classification: possibly performance-relevant because token inventory drives RPC work. Record request/chunk bounds and verify the selected-chain scan remains bounded; no broad synthetic benchmark is required.

No files or test paths are named. Start by reading PR #416 and the current execution tracked in #436, then review the existing direct-deposit explorer entry points and tests against the acceptance criteria. Done means the supported assets, bounded balance handling, stable selection, refresh behavior, displayed requirements, and required checks are covered.

Written by the indexing model from the issue text.