Warn before truncated value displays

[benma-agent]

Summary

• add a shared value confirmation helper that warns before UI label truncation
• route shared message verification, Ethereum tx/typed-data values, and BTC policy name/policy screens through it
• add boundary and ETH/BTC sign-message tests

Verification

• cargo fmt --manifest-path src/rust/Cargo.toml --all
• env PATH="$PWD/tools/prost-build-proto/target/debug:$PATH" cargo test --manifest-path src/rust/Cargo.toml -p bitbox02-rust --all-features -- --test-threads 1
• git diff --check

[cedwies]

utACK.

Nit: bitbox02-firmware/src/ui/components/label.h still has the stale comment // Keep this in sync with src/rust/bitbox02-rust/src/hww/api/ethereum.rs:MAX_CONFIRM_BODY_SIZE

Nit: BTC policy display now uses confirm_value() but there is no direct policy-specific long-policy test. But I leave that up to you if you want to add another test for this.