* fix(mobile): compare branches and show diffs with desktop's base-ref and whitespace settings
Base-ref resolution moves to src/shared and the host publishes the compare-against-upstream
and show-whitespace settings, so the phone's branch compare and diffs match desktop.
* fix(mobile): use the upstream compare policy on the review screen and rebuild diffs when whitespace changes
* fix(mobile): reread Source Control settings on Refresh and restart diffs loading under the old whitespace setting
* test(mobile): cover diffs dropped when desktop's whitespace setting arrives
* refactor(mobile): share one Source Control settings read per host across screens
