feat: Add deterministic Freighter browser mock for E2E testing

[Talentgift24-ux]

#783 [2026 Testing] Add a deterministic Freighter browser mock
Close #783

PR Title

feat: Add deterministic Freighter browser mock for E2E testing

Objective

Exercise connect, reject, sign, lock, account-change, and network-change flows for the Freighter wallet in CI.

Target Area

• tests/e2e/fixtures/freighter-mock.js
• tests/e2e/freighter.spec.js
• src/lib/wallet/freighter.js

Description

This PR introduces a deterministic browser mock for the Freighter wallet to facilitate robust End-to-End testing in CI. Previously, it was difficult to exercise wallet flows without manual interaction or flaky heuristics.

By injecting a lightweight mock via Playwright (page.addInitScript), we can accurately simulate the window.freighterApi interface and cover multiple edge cases.

Key Changes:

1. Mock Implementation (tests/e2e/fixtures/freighter-mock.js):
   • Exposes a standard Freighter API mock (isConnected, isAllowed, getAddress, signTransaction, etc.).
   • Provides a testing API (window.mockFreighter) to deterministically simulate accountChange, networkChange, lock states, and connection/signing rejections.
2. Automated E2E Tests (tests/e2e/freighter.spec.js):
   • Covers the primary flow (successful connection & address retrieval).
   • Covers failure paths (user rejects connection, wallet is locked).
   • Covers boundary cases (simulated network change and account change).
3. Wallet Connector Updates (src/lib/wallet/freighter.js):
   • Added listener handlers for freighterAccountChange, freighterNetworkChange, and freighterLock to gracefully support the mock and future-proof the connector for extensions that emit standard CustomEvents.

Acceptance Criteria Met

• Clear handling for invalid input, unsupported environments, and failure paths.
• Automated tests cover the primary flow, at least one boundary case (network/account change), and at least one failure case (reject/locked).
• User-facing documentation and developer guidance is updated inline and within the test suite to demonstrate mock usage.

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