Switch from postcard to cbor for the Bitcoin app

[bigspider]

This makes it easier to keep the wire protocol forward-compatible.

[Copilot]

Pull request overview

This PR switches the Bitcoin app/client wire protocol from postcard/serde serialization to CBOR via minicbor, aiming to make message evolution more forward-compatible through stable numeric indices.

Changes:

• Replaces serde derives with minicbor Encode/Decode derives across common message and error types.
• Updates app and client request/response serialization paths from postcard to minicbor.
• Adjusts affected response enum shapes and handler/test pattern matches.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
apps/bitcoin/common/src/message/mod.rs	Defines CBOR encoding for Bitcoin wire protocol message types.
apps/bitcoin/common/src/errors.rs	Adds stable CBOR indices for app error variants.
apps/bitcoin/common/Cargo.toml	Replaces runtime serde dependency with minicbor.
apps/bitcoin/client/src/client.rs	Switches client request serialization and response parsing to minicbor.
apps/bitcoin/client/Cargo.toml	Adds minicbor and removes direct postcard dependency.
apps/bitcoin/app/src/main.rs	Switches app request parsing and response serialization to minicbor.
apps/bitcoin/app/src/handlers/sign_psbt.rs	Updates PSBT signing response construction/tests for new response shape.
apps/bitcoin/app/src/handlers/get_master_fingerprint.rs	Updates master fingerprint response construction/tests for new response shape.
apps/bitcoin/app/Cargo.toml	Adds minicbor and removes postcard dependency.

Comments suppressed due to low confidence (1)

apps/bitcoin/common/src/message/mod.rs:104

• This field-bearing enum variant is not marked #[cbor(map)], so it is encoded positionally instead of with stable field keys. That leaves AccountCoordinates::WalletPolicy outside the documented forward-compatible encoding scheme and makes adding optional fields to this variant a breaking wire change.

    #[n(0)]
    WalletPolicy(#[n(0)] WalletPolicyCoordinates),

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