feat: AI preset generator, Spotify integration, community gallery API

AI Preset Generator (Butterchurn-resistant moat — needs compiler pipeline):
- assets/js/milkdrop/preset-prompt.ts — DSL grammar spec, prompt builder for
  LLMs. Specifies all 37 functions, 6 audio registers, 8 state slots,
  4 expression block types. Few-shot prompt format.
- assets/js/core/services/preset-generator.ts — Calls Workers AI or
  fallback procedural generator, compiles result through JIT pipeline,
  validates diagnostics.
- functions/api/generate-preset.ts — POST endpoint with Llama 4 Scout
  via Workers AI binding. Cleans LLM output, falls back to keyword-
  matched procedural preset if AI unavailable.

Spotify Web Playback SDK:
- assets/js/core/services/spotify-service.ts — PKCE OAuth flow, SDK
  lifecycle (idle→authorizing→connecting→ready), token/refresh
  management, player controls, createMediaElementSource audio routing.
- functions/api/spotify-token.ts — Proxies auth code and refresh token
  exchanges to Spotify API.
- assets/js/frontend/SpotifyPlayerUI.tsx — React component with connect
  button and now-playing controls.

Community Preset Gallery:
- functions/api/presets.ts — Full CRUD: list with search/tag/sort,
  single get with R2 .milk source, upload with R2+D1 storage. CORS.
- schema/d1-presets.sql — D1 schema with presets + favorites tables
  and indexes.
- wrangler.d1.example.toml, wrangler-secrets.example.json — config
  templates for D1, R2, and Spotify OAuth secrets.

Author: zz-plant

assets/js/core/services/preset-generator.ts

@@ -0,0 +1,78 @@
+import { compileMilkdropPresetSource } from '../../milkdrop/compiler.ts';
+import type { MilkdropCompiledPreset } from '../../milkdrop/types.ts';
+import { buildGeneratePrompt } from '../../milkdrop/preset-prompt.ts';
+
+export type GenerateStatus =
+  | { status: 'generating' }
+  | { status: 'compiling' }
+  | { status: 'ready'; preset: MilkdropCompiledPreset }
+  | { status: 'error'; message: string };
+
+export async function generatePreset(
+  description: string,
+  options: {
+    complexity?: 'simple' | 'moderate' | 'complex';
+    apiEndpoint?: string;
+  } = {},
+): Promise<MilkdropCompiledPreset> {
+  const endpoint = options.apiEndpoint || '/api/generate-preset';
+
+  const response = await fetch(endpoint, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({
+      description,
+      complexity: options.complexity || 'moderate',
+    }),
+  });
+
+  if (!response.ok) {
+    const err = await response.text();
+    throw new Error(`Generator API error: ${response.status} ${err}`);
+  }
+
+  const data = (await response.json()) as { milkSource: string };
+
+  const compiled = compileMilkdropPresetSource(data.milkSource, {
+    id: `ai-${Date.now()}`,
+    title: 'AI Generated',
+    origin: 'generated',
+  });
+
+  if (compiled.diagnostics.filter((d) => d.severity === 'error').length > 0) {
+    throw new Error(
+      `Generated preset has compilation errors: ${compiled.diagnostics.map((d) => d.message).join('; ')}`,
+    );
+  }
+
+  return compiled;
+}
+
+export async function generatePresetOnWorker(
+  description: string,
+  complexity: 'simple' | 'moderate' | 'complex' = 'moderate',
+): Promise<MilkdropCompiledPreset> {
+  buildGeneratePrompt(description, complexity);
+
+  const response = await fetch('/api/generate-preset', {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ description, complexity }),
+  });
+
+  if (!response.ok) {
+    const text = await response.text();
+    throw new Error(text);
+  }
+
+  const { milkSource } = (await response.json()) as { milkSource: string };
+
+  const sourceMeta = {
+    id: `ai-${Date.now()}`,
+    title: `AI: ${description}`,
+    origin: 'generated' as const,
+  };
+  const compiled = compileMilkdropPresetSource(milkSource, sourceMeta);
+
+  return compiled;
+}