feat(key-wallet): add normalize_phrase and is_word_in_language helpers

[llbartekll]

What

Adds two BIP39 input helpers to key-wallet's Mnemonic, plus a Language re-export, so downstream consumers can build recover-flow word/phrase validation on top of key-wallet instead of pulling in a second, direct bip39 dependency and re-deriving the logic.

• Mnemonic::normalize_phrase(&str) -> String — NFKD + lowercase + whitespace-collapse. Lenient input normalization for validation/membership of user-typed phrases; explicitly NOT BIP39 seed normalization (to_seed already performs the required NFKD for seed derivation).
• Mnemonic::is_word_in_language(&str, Language) -> bool — exact wordlist membership for a given language (no normalization; pre-normalize via normalize_phrase for case/accent tolerance).
• Re-export Language from the crate root + prelude (was only reachable via mnemonic::Language).
• Add the unicode-normalization dependency (already present transitively via bip39).

Why

In dashpay/platform PR #3842, rs-platform-wallet-ffi added recover-flow mnemonic helpers by depending directly on bip39 and re-implementing wordlist membership / phrase normalization. Review feedback flagged this as duplicating logic key-wallet already owns. These primitives let the platform crate become a thin facade over key-wallet — one source of BIP39 logic — and drop its direct bip39 dependency. App-specific concerns (CJK ideographic auto-split, the app's bundled-language policy) stay in the platform facade.

Tests

`cargo test -p key-wallet --lib mnemonic` adds `test_normalize_phrase`, `test_normalize_phrase_unicode_forms_converge`, `test_is_word_in_language` (36 mnemonic unit tests pass). `cargo clippy -p key-wallet` clean.

Notes

Branched off `3d0d5dc` (the commit dashpay/platform currently pins) for a minimal, conflict-free diff: only `key-wallet/{Cargo.toml, src/lib.rs, src/mnemonic.rs}` change.

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features
  • Added mnemonic phrase normalization with Unicode NFKD handling, automatic lowercasing, and whitespace collapsing for standardized input processing.
  • Added language-specific BIP39 wordlist validation for exact word membership verification.
  • Expanded public API to expose Language type alongside Mnemonic for language-specific operations.

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📥 Commits

Reviewing files that changed from the base of the PR and between 7ff6b24 and 307582c.

📒 Files selected for processing (3)

• key-wallet/Cargo.toml
• key-wallet/src/lib.rs
• key-wallet/src/mnemonic.rs

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.73%. Comparing base (3d0d5dc) to head (307582c).
⚠️ Report is 4 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #806      +/-   ##
==========================================
+ Coverage   72.55%   72.73%   +0.18%     
==========================================
  Files         322      323       +1     
  Lines       71006    71796     +790     
==========================================
+ Hits        51518    52224     +706     
- Misses      19488    19572      +84     

Flag	Coverage Δ
core	76.75% <ø> (+0.28%)	⬆️
ffi	45.89% <ø> (-0.53%)	⬇️
rpc	20.00% <ø> (ø)
spv	90.36% <ø> (+0.13%)	⬆️
wallet	71.37% <100.00%> (+0.08%)	⬆️

Files with missing lines	Coverage Δ
key-wallet/src/mnemonic.rs	95.14% <100.00%> (+0.59%)	⬆️

... and 36 files with indirect coverage changes