feat: workflow presets for bundled pipeline steps

[mvanhorn]

Summary

Adds reusable workflow presets that bundle pipeline steps (for example Detect+OCR, or Translate+Render), so a partial pipeline runs in one click instead of relying on the rigid Run All.

Why this matters

This maintainer-authored request (with community votes from RedRain77, Dear-moon, and TenviLi) asks for named step bundles, motivated by VRAM management (running the LLM and inpainting models in separate phases) and reviewability (getting clean inpainted images before translating) (#592). The backend already accepts an arbitrary ordered steps: string[] in StartPipelineRequest, and the toolbar's runStep helper already executes arbitrary chains, so this is a frontend-only addition that names and persists bundles.

A workflowPresets slice is added to the persisted zustand preferencesStore (mirroring the existing favoriteFonts pattern) with addWorkflowPreset/removeWorkflowPreset/renameWorkflowPreset actions and two built-in defaults. CanvasToolbar.tsx gets a presets dropdown that composes the corresponding chain pickers and calls the existing runStep with the deduped step list, reusing getConfig() engine-id resolution so no engine ids are hardcoded. A small dialog creates, names, and deletes presets. No Rust, RPC, or generated-schema changes.

Changes

• workflowPresets slice + actions in preferencesStore, persisted, with seeded defaults.
• Presets dropdown in CanvasToolbar that runs the composed step chain.
• Create/name/delete dialog.

Testing

• Store round-trips presets through persistence; add/remove/rename behave as expected.
• Selecting a preset runs the deduped step chain.

Refs #592

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