Fix mnemonic word deselection

[Jim8y]

Problem

Deselecting a mnemonic word removed one character instead of the selected word. Matching by text also could not distinguish repeated-word buttons.

Change

Track selected button instances in order and rebuild the phrase from the selected words. Removing or reselecting one occurrence no longer damages another occurrence or a substring.

This independent PR addresses audit P2-01 only, based on master 623603d.

Validation

• 4 regression tests passed; three failed with the original implementation.
• iOS 26.5 iPhone 17 simulator and Android API 36 arm64 emulator: exercised the actual VerifyMnemonicPage controls and handlers. Both passed six checks for whole-word removal, middle-word removal, reselection, independent duplicate occurrences, the complete phrase, and the production validator.
• Used a public synthetic test phrase; wallet creation was disabled in the external fixture. No real seed or funded wallet was accessed.
• Removed the fixture and rebuilt/installed the normal product on both platforms; startup smoke passed with zero build warnings/errors.
• Android screen protection remained enabled; the protected mnemonic screen was not used for the evidence screenshot. The visible result summary was captured instead.

Screenshots, native result JSON and simulator-only fixture code remain outside the repository.

[Copilot]

🟢 Approval recommended

The fix is localized to the click handler, removes the confirmed faulty substring/duplicate-word behavior, and is backed by focused regression tests.

Pull request overview

Fixes mnemonic-word deselection in VerifyMnemonicPage by tracking selected button instances (not text) and rebuilding the phrase from the selected words, which correctly handles whole-word removal and repeated-word buttons.

Changes:

• Replace text-based removal logic with instance-based selection tracking and phrase rebuild.
• Add a dedicated regression test project (with lightweight MAUI stubs) covering deselection, middle-word removal, duplicates, and full selection order.
• Register the new test project in the solution.

File summaries

File	Description
OneGateApp/Pages/VerifyMnemonicPage.xaml.cs	Switch selection state to an ordered List<Button> and rebuild the mnemonic phrase from selected instances.
tests/P2-01/MnemonicSelectionTests.cs	Adds regression tests that reproduce the prior bug and validate correct behavior (including duplicate word instances).
tests/P2-01/PageStubs.cs	Provides minimal MAUI/page/service stubs so the production click handler can be compiled and exercised in tests.
tests/P2-01/MnemonicSelection.Tests.csproj	New xUnit test project wiring in the production VerifyMnemonicPage.xaml.cs plus test dependencies.
tests/P2-01/README.md	Documents how to run the regression tests and what they cover.
OneGateApp.slnx	Adds the new regression test project to the solution.

Review details

• Files reviewed: 6/6 changed files
• Comments generated: 0
• Review effort level: Lite

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