test(repository): settings_repository (+16 tests)

[TaprootFreak]

Summary

Stage 24 of the coverage push. The `SettingsRepository` wraps SharedPreferences and is one of the few storage-adjacent files that can be tested directly via `SharedPreferences.setMockInitialValues`.

Group	Cases
currentWalletId	3
language	3
currency	3
terms (terms + softwareTerms)	3
networkMode	4

What each group covers

• currentWalletId: null when not stored; `saveCurrentWalletId` persists; `removeCurrentWalletId` clears.
• language: falls back to `'en'` for non-German system locales (PlatformDispatcher locale in tests is en_US); returns stored value when set; setter persists.
• currency: defaults to `'CHF'` when not stored; returns stored value; setter persists.
• terms: both `termsAccepted` and `softwareTermsAccepted` default to false; setters persist; setting one does not flip the other (independent flags).
• networkMode: defaults to mainnet when no value AND when stored value is unknown (`firstWhere`'s `orElse`); reads via the enum constructor argument name (`'Mainnet'` / `'Testnet'`) — not the Dart enum identifier (this trap surfaced during the tests because the production code uses `.name` and the constructor arg shadows it); setter persists.

Test plan

• `flutter analyze` clean
• `flutter test` — 16 / 16 passing locally
• CI green