fix(ui): avoid mobile selection action overlap (#622)

## Summary
- place CodeNomad selection actions below the final selected line on
touch-only devices
- keep desktop and hybrid fine-pointer positioning unchanged
- fall back to the message stream's top edge when bottom placement would
be clipped

## Validation
- 146 UI Node tests
- npm run typecheck
- npm run build --workspace @codenomad/ui
- git diff --check

Closes #598

.github/workflows/pr-build.yml

@@ -106,6 +106,7 @@ jobs:
           node --import tsx --test
           packages/ui/src/components/session-list-visibility.test.ts
           packages/ui/src/lib/hooks/use-app-session-capture.test.ts
+          packages/ui/src/lib/message-selection-position.test.ts
           packages/ui/src/lib/trailing-resync.test.ts
           packages/ui/src/stores/abort-created-workspace-cleanup.test.ts
           packages/ui/src/stores/app-session-reconciliation.test.ts