feat(smart-crop): overhaul with face detection, social presets, and 3 modes

Replace the confusing 2-mode smart crop with a clear 3-mode system:
- Subject Focus: Sharp attention/entropy saliency crop with social media presets
- Face Focus: MediaPipe face detection with headshot framing presets
- Auto Trim: Border removal with optional pad-to-square

Adds detectFaces() to AI package, face preset constants, backward
compatibility for old mode names, and comprehensive integration tests.

packages/ai/src/face-detection.ts

@@ -1,4 +1,5 @@
-import { readFile, writeFile } from "node:fs/promises";
+import { readFile, unlink, writeFile } from "node:fs/promises";
+import { tmpdir } from "node:os";
 import { join } from "node:path";
 import { type ProgressCallback, runPythonWithProgress } from "./bridge.js";
 
@@ -7,6 +8,10 @@ export interface BlurFacesOptions {
   sensitivity?: number;
 }
 
+export interface DetectFacesOptions {
+  sensitivity?: number;
+}
+
 export interface FaceRegion {
   x: number;
   y: number;
@@ -20,6 +25,11 @@ export interface BlurFacesResult {
   faces: FaceRegion[];
 }
 
+export interface DetectFacesResult {
+  facesDetected: number;
+  faces: FaceRegion[];
+}
+
 export async function blurFaces(
   inputBuffer: Buffer,
   outputDir: string,
@@ -48,3 +58,32 @@ export async function blurFaces(
     faces: result.faces ?? [],
   };
 }
+
+export async function detectFaces(
+  inputBuffer: Buffer,
+  options: DetectFacesOptions = {},
+  onProgress?: ProgressCallback,
+): Promise<DetectFacesResult> {
+  const inputPath = join(tmpdir(), `detect_faces_${Date.now()}.png`);
+
+  try {
+    await writeFile(inputPath, inputBuffer);
+    const { stdout } = await runPythonWithProgress(
+      "detect_faces.py",
+      [inputPath, "unused", JSON.stringify({ ...options, detectOnly: true })],
+      { onProgress },
+    );
+
+    const result = JSON.parse(stdout);
+    if (!result.success) {
+      throw new Error(result.error || "Face detection failed");
+    }
+
+    return {
+      facesDetected: result.facesDetected,
+      faces: result.faces ?? [],
+    };
+  } finally {
+    await unlink(inputPath).catch(() => {});
+  }
+}

packages/ai/src/index.ts

@@ -1,6 +1,7 @@
 export { removeBackground } from "./background-removal.js";
 export { isGpuAvailable, shutdownDispatcher } from "./bridge.js";
-export { blurFaces } from "./face-detection.js";
+export type { DetectFacesResult, FaceRegion } from "./face-detection.js";
+export { blurFaces, detectFaces } from "./face-detection.js";
 export { inpaint } from "./inpainting.js";
 export { extractText } from "./ocr.js";
 export { seamCarve } from "./seam-carving.js";