feat(web): use OKLCH for theme palettes - #6036

💭 I'm reviewing the ThemeLibraryStorageError changes to verify the earlier issues were resolved—the artificial Error construction and the read failure leaking into write messages both appear to be fixed now. I need to check whether we're double-encoding the same distinction with both a specific error tag and a single-value operation or reason literal. Looking at the schema,… Now I'm checking whether the reason field on ThemeLibraryStorageError is actually being used for control flow or user-facing messages, since both operation and reason are multi-value enums and I want to make sure we're not encoding the same distinction twice.… Looking at the other changed files to see if they follow Effect conventions—themePalette.ts only uses Schema without any ManagedRuntime or Layer setup, and the test files don't appear to create or consume Effect services either. There's a potential concern starting to form here.… Checking whether the reason field might be redundant — for write operations it's always "storage-unavailable", but for read operations it can vary, so there's no actual duplication happening.…