Follow-Up to #40: Restore Backspace bulk deletion

[alvst]

Summary

Completes the current multi-select keyboard workflow without reintroducing the older selection UI.

• Backspace now deletes the current history or Pinboard selection when no search query is being edited.
• Backspace continues to edit a nonempty search query first.
• Paste Stack Backspace behavior remains limited to its selected stack entry.
• The existing Cmd-click, Shift-click, Forward Delete, source scoping, selection repair, and contextual deletion stay unchanged.

Why this is separate

The original multi-select PR is already represented in the modern implementation. This is the small keyboard regression left behind after the later interaction work.

Dependency

This draft is based on #55 and should merge after it. It will be rebased onto main once its dependencies land.

Validation

• swift build
• swift test — 7 tests passed
• git diff --check

Screenshot

No visual change beyond the existing selected-card state. Any later visual QA will use Debug Demo only (Pesty --demo).