test: add Windows desktop E2E smoke harness

[Azganoth]

Summary

• Add an isolated Tauri build flavor that enables the WebdriverIO plugins, capabilities, frontend bridge, application identifier, and target directory only for desktop E2E runs.
• Add a single-worker Windows-local smoke test that drives Help → Diagnostics through semantic UI selectors, verifies real Tauri IPC and the E2E application identity, and uses a deterministic 1024×768 outer window.
• Retain correlated runner, frontend, and backend logs for every run plus a screenshot, diagnostics, test error, and compact semantic UI snapshot on failure.
• Document local setup, .env fault injection, state isolation, port use, artifact sensitivity, and the assembled-desktop verification boundary.

Related Issue

Closes #179

Verification

• pnpm check — passed for the cross-cutting frontend, Rust, configuration, documentation, and dependency changes.
• pnpm test:e2e:desktop — built and launched the isolated application and passed the Help → Diagnostics smoke workflow against real IPC.
• LEAFDOWN_E2E_FORCE_FAILURE=1 pnpm test:e2e:desktop — returned a failing test while retaining the expected evidence with no artifact-capture errors.
• Confirmed passing and forced-failure runs left no leafdown-e2e process or listener on port 4445.
• Confirmed the normal application build excludes the E2E identifier, capabilities, frontend bridge, and WDIO Rust plugins.
• pnpm exec oxfmt --check e2e/desktop/wdio.conf.ts, pnpm exec oxlint --deny-warnings e2e/desktop/wdio.conf.ts, pnpm lint:tsc, and git diff --check origin/main...HEAD — passed after the deterministic window-size change.

Not verified: Linux or macOS execution and CI-hosted desktop execution; the initial harness is intentionally Windows-local and excluded from pnpm check.

Notes

• The 1024×768 setting controls the outer native window. The captured WebView on the verified Windows host is 1008×759 because the operating-system frame consumes part of that size.
• E2E state uses com.azganoth.leafdown.e2e, so persisted window state and preferences do not affect the ordinary Leafdown application.
• Additional workflows, visual regression, parallel workers, CI integration, and the related Project drafts remain out of scope.