fix: stabilize Flatpak WebKit runtime

[vanloctech]

Summary

• move the Flatpak runtime from GNOME Platform 50 to 49 while investigating the WebKitGTK 2.52 launch crash reported in Crash on launch (SIGABRT in webkit_website_data_manager_get_cookie_manager) on openSUSE Tumbleweed - AppImage and Flatpak #119
• add a real Xvfb launch smoke test for the Flatpak bundle in the dedicated test workflow and release workflow
• run the Flatpak test workflow on relevant pull requests so future runtime regressions are caught before release

Validation

• bun run biome check --write .
• bun run tsc -b
• cargo check
• pre-commit hook (102 tests)
• GitHub Actions will build and launch the Flatpak bundle

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Include frontend build inputs in the Flatpak PR paths

A PR that changes the React bundle or its build inputs (src/**, package.json, bun.lock, Vite configuration, etc.) matches none of these paths, so this workflow is skipped even though src-tauri/tauri.conf.json runs bun run build and packages ../dist into the .deb converted by this job. Such changes can introduce a packaged-app launch regression but will now be detected only after merging to main or during a tagged release; include the frontend/build inputs here or remove the restrictive filter.

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