Tests: Implement Shift+Arrow selection extension (skipped unit tests)

[jjeff]

Problem

Two unit tests in `tests/unit/keyboard-manager.test.ts` are `it.skip`:

• Line 340: "should extend selection with Shift+ArrowDown"
• Line 356: "should extend selection with Shift+ArrowUp"

This is a standard keyboard multi-select pattern and should work.

Acceptance criteria

• Shift+Arrow extends the current selection range.
• Both unit tests un-skipped and passing.
• E2E coverage in `tests/e2e/keyboard-navigation.spec.ts`.