Fix filter dropdowns returning empty string for All option

Problem: Status and Priority filter dropdowns return empty string when 'All' is selected, causing parse error.

Location: Omni/Jr/Web.hs - taskListHandler and form rendering

Root Cause: FromHttpApiData instances in Omni/Task/Core.hs return Left '' for empty strings. Servant treats this as parse failure rather than Nothing.

Fix Options: A) Change FromHttpApiData to return sentinel for empty (breaks semantics) B) Use custom query param parsing - treat empty as Nothing

Note: emptyToNothing helper exists but only applied to maybeNamespace. Need to verify if Servant's query param parsing calls FromHttpApiData before handler runs.

Actual fix: Either make FromHttpApiData succeed on empty, or use QueryParam' '[Optional] 'status' Text and parse manually in handler.