test(widgets): mnemonic extensions + controller styling (+9 tests)

[TaprootFreak]

Summary

Stage 44 of the coverage push. Pure-function tests for the seed-word string utilities and the BIP39 highlighting on the mnemonic input controller.

Cases

Target	Cases
`SeedStringExtension.seedWords`	5 — splits on single spaces; collapses repeated whitespace + tabs; strips leading/trailing whitespace; treats newlines as whitespace; empty input returns empty list
`MnemonicListExtension.seed`	2 — joins controller texts and trims each entry; preserves the controller order
`MnemonicInputFieldController.buildTextSpan`	2 — base style for a word in the BIP39 list; merges in the red non-match style for an unknown word

What's pinned

• The seed-word split uses `RegExp(r'\s+')`, so any whitespace boundary (tab / newline / multiple spaces) counts — all four whitespace classes are exercised.
• `MnemonicListExtension.seed` trims each controller entry individually before joining, so accidental leading/trailing spaces (e.g. from a paste) don't bleed into the recovered phrase.
• `buildTextSpan` differentiates valid vs invalid BIP39 words via a style merge — a word that is in the wordlist returns the unmodified base style, anything else picks up the red status color. Pinned by both branches.

Test plan

• `flutter test test/widgets/mnemonic_extensions_test.dart` — 9 pass
• `flutter analyze` clean on the new file
• CI green