Align Trezor dev tooling with bitkit-docker

[ovitrif]

Related: bitkit-ios#563 feat: add Trezor emulator bridge, bitkit-docker#7 feat: add trezor emulator, #998 epic: Trezor Hardware Wallet UI

Android Trezor Bridge support (#792) landed before bitkit-docker shipped its Trezor User Env, so two pieces stayed behind while iOS builds on bitkit-docker end to end. The runtime path already works against bitkit-docker (Bridge port 21325 is kept for Bitkit), this is about closing the remaining tooling gap.

Tasks

• Electrum override: iOS has TREZOR_ELECTRUM_URL to point Trezor flows at the local regtest Electrum from bitkit-docker in Debug builds. The Android watcher resolves Electrum via Env.electrumUrlForNetwork, which on dev builds returns the staging regtest server; the local one is only used by E2E local-backend builds. Add an equivalent env-driven override following the TREZOR_BRIDGE / TREZOR_BRIDGE_URL build config pattern, mirrored in .env.example.
• Docs: the only Android emulator setup instructions live in the pre-bitkit-docker PR bodies of #792 and #939 (manual trezor-user-env clone, run.sh, SDL3 workaround). Add a short doc in docs/ pointing to the scripts/trezor-emulator helper (start, adb) and the TREZOR_BRIDGE build flags, consistent with .agents/journeys/README.md.