Improve link block editing

[oobagi]

Summary

• move Link block editing into a footer prompt backed by shared single-line input behavior
• add paste, word/line editing, and delete parity for Link prompts and browser inputs
• improve Link card rendering and wrapping, with regression coverage

Validation

• go test ./internal/ui ./internal/editor ./internal/browser ./internal/format
• env HOME=$(mktemp -d) go test ./...
• go vet ./...