Refactor display of unnamed morphisms in frontend

[epatters]

Our system for printing objects, used particularly to print unnamed morphism generators, suffers from several problems:

• Redundant code across TS and Rust with overlapping but different behavior: morLabelOrDefault in frontend vs mor_generator_dom_cod_label_strings and ob_label_string in catlog-wasm
• Ambiguity about intended output format: the Rust functions are ultimately used in a LaTeX math display, but that isn't documented at the source and indeed the current implementation does not correctly escape long labels by enclosing them in \text{...}
• Display implementations tacitly special cased to particular logics: see this FIXME

This issue is follow up to #1091 but much of the problem predates it.