FFI methods panic on error instead of returning `Result`

[AlexAxthelm]

Background

The UniFFI/WASM FFI boundary in shared/src/ffi.rs currently uses
panic! to handle deserialization and serialization errors. Panicking
across an FFI boundary causes the host process (Swift/JS) to hard-crash
with no opportunity for error recovery.

What to change

Refactor the FFI methods to return Result<Vec<u8>, SomeError> instead
of panicking. UniFFI has native support for Result-returning functions,
so this should be straightforward to wire up on both the Rust and Swift
sides.

Scope

• Define an appropriate error type (or reuse one from crux_core)
• Update FFI method signatures in shared/src/ffi.rs
• Update the Swift core.swift glue to handle the returned Result
• Add tests covering the error path

References

• Flagged by Copilot code review on PR Initial app #2
• UniFFI Result support: https://mozilla.github.io/uniffi-rs/udl/errors.html