feat: add system test mode support with configurable import/export paths

[clFaster]

Summary

This PR introduces a full system testing foundation for the Tauri app and wires it into CI.

1) System test framework

• Added WebdriverIO + tauri-driver setup for real desktop/system tests.
• Added deterministic test-mode behavior (TAURI_TEST_MODE) for test stability.
• Added stable UI selectors (data-testid) for key board/settings interactions.
• Added deterministic import/export path handling for tests.
• Added isolated test data storage for system tests.

2) Core system scenarios

Implemented end-to-end scenarios in e2e/specs/system.e2e.mjs:

• Smoke flow: create board + open/close settings.
• Board lifecycle: create, rename, duplicate.
• Board persistence across restart.
• Settings persistence across restart.

3) CI integration

• Added dedicated system-tests job in pr-verification.yml.
• Installs and validates tauri-driver.
• Ensures Edge WebDriver availability with fallback logic:
  • use existing msedgedriver if present,
  • check common preinstalled locations,
  • fallback download from multiple hosts.
• Uploads system test artifacts on failure (.wdio-artifacts, .system-test-data).

Out of Scope

• Unit tests (planned separately, intentionally not included in this PR).