Add l_ui_add_body_class helper for composable body modifiers

[thomasgallagher]

Replaces content_for :l_ui_body_class with an accumulating helper so a shared layout and an individual page can each contribute body modifier classes without clobbering one another. The legacy content_for is still honoured as a temporary bridge.

[claude]

Review

Replaces the single content_for :l_ui_body_class slot with an accumulating l_ui_add_body_class helper so multiple templates in the same request can each contribute body modifier classes without clobbering one another; the legacy content_for path is kept as a deprecated bridge.

No issues found. The implementation is correct: @_l_ui_body_classes is an instance variable on the shared view context so it accumulates naturally across nested templates, token_list (Rails 7.1+, safe given the ~> 8.0 Rails pin) handles deduplication and nil-safety, and the legacy bridge reads content_for without consuming it so calling l_ui_body_classes is idempotent. Tests cover all documented behaviours.

---

✅ Ship