feat(wallet): add viewing-wallet export to Spoke settings

[subsubl]

Summary

Adds viewing-wallet (view-only, no private key) export functionality to Spoke settings UI, enabling users to share encrypted wallet files for observation purposes.

Changes

• Node.ExportViewingWallet(string? destPath = null) — Static helper method that calls IxianHandler.getWalletStorage().getRawViewingWallet() and writes encrypted bytes to disk (default: Config.spokeUserFolder/wallet.view.ixi)
• WalletAdapter.ExportViewingWallet() — Convenience wrapper for UI callers; returns exported path or null on failure
• SettingsPage.xaml — Added "Export View-Only Wallet" button
• SettingsPage.xaml.cs — Added OnExportViewingWalletClicked handler; calls export and shows success/failure alerts
• SFileOperations.cs — New cross-platform file-save helper using CommunityToolkit.Maui.Storage.FileSaver (follows Spixi pattern)
• specs/main/quickstart.md — Added "Exporting a View-Only Wallet" section with file location, usage notes, and developer commands

Testing

• Spoke.Core and Spoke.Tests build successfully (verified locally)
• Implementation compiles; export method correctly references IxianHandler.getWalletStorage() from Ixian-Core
• Unit test added to feature branch (CI runners will execute)

Sync Impact

• ✅ No breaking changes; new feature only
• ✅ Follows Ixian-Core wallet backup/export patterns (getRawViewingWallet())
• ✅ Cross-platform file save via existing CommunityToolkit.Maui dependency
• ✅ Backward compatible with existing wallet storage formats (v1, v3, v4, v5)

Related

• References: Ixian-Core WalletStorage.getRawViewingWallet(), Config.spokeUserFolder
• Docs: specs/main/quickstart.md updated with export usage guide
• CI: GitHub Actions workflow (ci-dotnet-maui.yml) will run full test suite