Add post-processing audio effects system

Adds a full effects pipeline powered by Spotify's pedalboard library,
enabling users to apply professional DSP effects (flanger, reverb, delay,
compressor, pitch shift, filters, gain) to generated audio.

Key features:
- Effects chain editor with drag-and-drop reordering (dnd-kit)
- Generation versions: clean copy always saved, processed versions created on top
- Built-in presets (Robotic, Radio, Echo Chamber, Deep Voice) + custom user presets
- Per-profile default effects chain (auto-applied to new generations)
- Per-generation effects override from the generation form
- Apply effects to existing generations from history (creates new version)
- Ephemeral preview endpoint for auditioning effects without persisting
- Dedicated Effects sidebar tab with preset management and live preview
- Version switcher in history cards with expandable panel
- Backward-compatible: existing generations backfilled as clean versions

app/src/stores/effectsStore.ts

@@ -0,0 +1,26 @@
+import { create } from 'zustand';
+import type { EffectConfig } from '@/lib/api/types';
+
+interface EffectsStore {
+  selectedPresetId: string | null;
+  setSelectedPresetId: (id: string | null) => void;
+
+  // Working chain for the detail panel (editing a preset or building a new one)
+  workingChain: EffectConfig[];
+  setWorkingChain: (chain: EffectConfig[]) => void;
+
+  // Track if editing an existing preset vs creating new
+  isCreatingNew: boolean;
+  setIsCreatingNew: (v: boolean) => void;
+}
+
+export const useEffectsStore = create<EffectsStore>((set) => ({
+  selectedPresetId: null,
+  setSelectedPresetId: (id) => set({ selectedPresetId: id, isCreatingNew: false }),
+
+  workingChain: [],
+  setWorkingChain: (chain) => set({ workingChain: chain }),
+
+  isCreatingNew: false,
+  setIsCreatingNew: (v) => set({ isCreatingNew: v, selectedPresetId: v ? null : null }),
+}));