feat(frontend): add access to the imported wallet's signing canister

[sbpublic]

Motivation

PR1 displays what an imported wallet holds; PR2 moves the ICP and ICRC balances. The remaining chains — BTC, EVM, SOL — cannot be moved the same way, because their keys are not derived from the seed phrase at all.

They are chain-key (threshold) keys rooted in the source wallet's own helper canister. On the management canister, ecdsa_public_key and schnorr_public_key accept a canister_id argument, but sign_with_ecdsa and sign_with_schnorr do not — the key is derived from the caller. So those addresses are readable by anyone (which is how PR1 derives them offline) and signable only by that canister.

This PR adds the access layer for asking it to sign. No user-visible change; the per-chain send flows follow in their own PRs, one per chain.

Stacked on #13648; review that first. This PR targets its branch.

Changes

• Vendor the helper canister interface following the existing third-party pattern (xtc_ledger, icrc3, …): a build script produces the .did, dfx.json points at it, generate.sh runs it, and the bindings are generated rather than hand-written.
• Add the canister wrapper and api layer for the four signing methods the remaining chains need: send_eth, send_erc20, send_btc, sign_sol.

Four decisions worth review attention:

• The candid is fetched from the canister, not downloaded from a repo. Every other script here downloads from a public URL; this interface is published nowhere. The canister exposes it via __get_candid_interface_tmp_hack, which makes the deployed canister the only authoritative source — and fetching from it keeps the checked-in bindings honest about what is live, rather than a snapshot transcribed from the extension's minified bundle. The fetch fails the build if the reply does not declare a service, so a truncated response cannot silently produce bindings for an empty interface.
• { Sign: null } everywhere, never Send. The canister will broadcast for us; we don't let it. Broadcasting ourselves means we see the transaction hash and the RPC error directly, and a third party stays off the critical path for everything but the signature.
• Not routed through CanisterApi. That cache holds one actor per principal in a module-level map that is never evicted, which would pin the imported identity — and the key derived from the seed phrase — for the lifetime of the page, outliving both the reset button and the send itself. Sends are rare enough that building the actor per call costs nothing worth keeping a secret around for.
• plug_helper added to the did.delete.types allowlist. That script deletes any generated folder not named in a hardcoded list; without the entry the bindings are removed at the end of the pipeline.

This touches dfx.json and scripts/, which AGENTS.md marks as restricted — done with explicit approval, and following the established pattern rather than inventing one.

Tests

New plug-helper.canister.spec.ts (9 cases): each method returns its payload, throws the canister's error text on Err, and — for eth and erc20 — is asserted to pass arguments in the canister's exact positional order and to request { Sign: null } rather than Send. The positional assertions are the valuable ones: the interface is all unnamed nat parameters, so a transposed gas and nonce would type-check and fail only on chain.

Local gates, all green:

Gate	Result
npm run format	clean
npm run lint -- --max-warnings 0 (whole repo)	clean
npm run check	5891 files, 0 errors
npm run check:tests	7119 files, 0 errors
targeted vitest	9 passed

Regenerating exposed one wrinkle worth recording: did.update.types.mjs rewrites 15 unrelated *.factory.certified.did.js files with a cosmetic trailing comma, which the pipeline's final npm run format cleans up. Running the steps by hand without that final format would have committed 15 files of churn; the diff here touches none of them.

Not verified against the live canister. eth_address confirmed the canister accepts callers other than the source wallet, but no signing method has been called by an outside caller. That is the main risk in this stack and is tested in the EVM PR, which is the first to exercise a real signature.

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