fix(escrow): require configuration before deposits

[prissca]

Closes #204

Summary

• fail closed with escrow_not_configured when a deposit targets an unconfigured queue
• make get_config use the same explicit unconfigured behavior instead of returning a misleading sentinel admin
• add regression coverage for the depositor-as-admin attack setup and unconfigured reads

Verification

• cargo fmt --all -- --check
• git diff --check
• full cargo test -p lineproof-escrow is left to CI because this Windows host has the Rust MSVC target but no link.exe

Design note

This keeps configuration queue-scoped. A contract-wide default admin would create a second source of authority that must be migrated and governed; requiring an explicit queue configuration makes the trust boundary visible and prevents accidental deployment in an implicit self-admin state.

[vercel]

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