fix(desktop): open message links with OS browser

[taner-caliskan]

Summary

• Route message-link left clicks through Tauri's opener instead of relying on WebKitGTK handling target="_blank".
• Reuse the same OS-opener path for the existing “Open link” context-menu action.
• Add an E2E regression proving the click reaches the native opener bridge without in-webview navigation.

Root cause

The markdown anchor relied on native webview target="_blank" behavior. The current Fedora AppImage/WebKitGTK runtime does not reliably hand that navigation to the default browser, so clicking appeared to do nothing. The Tauri opener plugin was already permitted and used for the context-menu action.

Verification

• pnpm test — 3,769 passed
• pnpm typecheck — passed
• pnpm check:file-sizes — passed
• targeted Biome check — passed
• pnpm build:e2e — passed

The new Playwright test is included in the smoke project. Local execution is blocked by a Chromium crash in this host's Playwright environment before channel selection; the production E2E build itself passes.

Risk

Low. This changes only external markdown-link activation and retains the existing Tauri opener error toast.