refactor: eliminate local state mirroring, use TanStack Query as single source of truth

[saraichinwag]

Summary

Eliminates local state mirroring patterns that bypass TanStack Query's cache, addressing the root cause of race conditions like #262 (blank fields on initial load).

Closes #263

Changes

PipelineContextFiles.jsx

• Removed all manual state (files, loading, error, uploading, deleting) and the loadFiles fetch-in-useEffect pattern
• Now uses useContextFiles(pipelineId), useUploadContextFile(), and useDeleteContextFile() — TanStack hooks that already existed but were unused
• TanStack Query handles caching, loading states, and automatic refetch after mutations

InlineStepConfig.jsx

• Replaced useEffect-based initialization of localValues with useMemo to derive initial values from handlerConfig prop + field schema
• Replaced raw updateFlowStepConfig() API call with new useUpdateFlowStepConfig mutation that properly invalidates flow query cache on success
• Local state for controlled inputs is preserved (needed for typing without per-keystroke cache updates)

FlowScheduleModal.jsx

• Replaced manual intervals fetch-in-useEffect + useAsyncOperation with new useSchedulingIntervals() TanStack query hook
• Intervals are cached with staleTime: Infinity since they don't change at runtime

New hooks

• useUpdateFlowStepConfig in queries/flows.js — mutation for flow step config updates with cache invalidation
• useSchedulingIntervals in queries/config.js — query for scheduling interval options

FlowStepCard.jsx

• Passes pipelineId and flowId to InlineStepConfig for proper cache invalidation

Not changed (lower priority)

• PromptField.jsx — pattern is mostly correct (local state for controlled input + useEffect sync from prop is fine)
• QueueablePromptField.jsx — already uses TanStack mutations for queue saves

Testing

• Pure internal refactor — no visual or behavioral changes
• All imports/exports verified consistent