Add multi-select bulk deletion

[alvst]

What changed

• Adds Finder-style clip selection: normal click selects one item, ⌘-click toggles individual items, and ⇧-click selects the contiguous range from the selection anchor.
• Visually highlights every selected card and exposes a Delete N bulk action.
• Makes ⌘⌫ and Forward Delete remove the current selection.
• Keeps bulk deletion scoped to the active history or pinboard collection, so it never erases saved copies elsewhere.
• Keeps contextual Delete intuitive: it deletes the whole current selection when invoked from a selected card, otherwise just that card.

Why

This enables selecting a contiguous range or an arbitrary set of clipboard entries before deleting them in one action without treating pinboards as disposable history.

Validation

• swift build
• VERSION=1.0.0 BUILD=1 ./scripts/build_app.sh
• git diff --check
• Isolated Swift harness covering normal, ⌘, ⇧, and bulk-delete selection behavior.

Screenshots

Demo mode — Pesty window only.