feat(config): extract STYLE_EXTRACTION_PROMPT to shared lib for client reuse

Signed-off-by: baizhi958216 <1475289190@qq.com>

lib/styleExtraction.ts

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+export const STYLE_EXTRACTION_PROMPT = `You are a senior concept artist helping describe an image's visual style so that a text-to-image diffusion model (FLUX) can reproduce the same aesthetic on different subjects.
+
+Look at the attached image and produce a single English style-prompt string that captures ONLY its visual style — NOT its subject matter. Focus on:
+- Medium / technique (e.g., watercolor, oil painting, cel-shaded anime, 3D render, pixel art)
+- Line work and rendering (sharp ink outlines, soft shading, painterly brushstrokes, flat colors)
+- Color palette and lighting (pastel, saturated, monochrome, warm golden-hour, cool neon, high contrast)
+- Mood and atmosphere (dreamy, melancholic, cinematic, nostalgic, gritty)
+- Any recognizable artistic influence (Ghibli, Makoto Shinkai, ukiyo-e, vaporwave, cyberpunk anime, etc.)
+
+Do NOT describe the characters, objects, or scene contents. Output exactly one JSON object:
+{"stylePrompt": "<comma-separated English visual-style attributes, ~30-60 words>"}`;