fix(refresh): preserve article selection while refreshing

[marcomarcogd]

Description

Keeps the currently open article available while a background feed refresh replaces the first article page. This prevents the article list from flashing empty and the detail pane from becoming blank when the selected article is not present in the refreshed first page.

Type of Change

• 🐛 Bug fix (non-breaking change which fixes an issue)
• ✅ Test addition/update

Related Issues

Fixes #874

Changes Made

• Add a preserve-selection mode to article fetching.
• Keep the selected article exactly once when it falls outside the refreshed first page.
• Use preserve-selection mode only for background refresh and FreshRSS completion paths.
• Add unit coverage for preserved, already-present, and unselected cases.

Testing

Test Configuration

• OS: macOS
• MrRSS Version: current main
• Node Version: 26.7.0

Test Steps

1. NODE_OPTIONS=--localstorage-file=/private/tmp/mrrss-vitest-874-localstorage npm test -- --run src/App.test.ts
2. npm run lint
3. npm run build
4. git diff --check

All targeted checks pass.

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• My changes generate no new warnings
• I have added tests that prove my fix is effective
• New and existing targeted tests pass locally

Breaking Changes

None.