refactor(cams): centralize the UserTaskState → UI mapping

[Zeroupper]

Problem

(verified) lib/ui/pages/task_list_page.dart hardcodes a Map<UserTaskState, Icon> (:742-751) plus ~8 scattered state comparisons (e.g., :202-203, :337-339, :434-435); a CAMS enum change degrades silently (missing map entries) rather than failing loudly.

Proposed fix

One exhaustive switch (Dart 3 exhaustiveness gives compile errors on enum drift) in a single presentation mapper used by all task UI.

Acceptance criteria

• One mapping site
• Compile-time error if CAMS adds/renames a state

Effort: S · Priority: P2 · Change risk: Low