feat(headroom): add extras detection and install UI (#2403)

- add Headroom extras status + install endpoints
- show Headroom version + code/ml extras in Token Saver UI
- fix Windows interpreter selection to read from env with headroom-ai

src/app/api/headroom/extras/route.js

@@ -0,0 +1,30 @@
+import { NextResponse } from "next/server";
+import { findPython310, getInstalledHeadroomExtras, HEADROOM_COMPRESSION_EXTRAS } from "@/lib/headroom/detect";
+import { installHeadroomExtras } from "@/lib/headroom/process";
+
+export const dynamic = "force-dynamic";
+
+export async function GET() {
+  try {
+    const python = findPython310();
+    const status = getInstalledHeadroomExtras(python);
+    return NextResponse.json({
+      available: HEADROOM_COMPRESSION_EXTRAS,
+      ...status,
+    });
+  } catch (error) {
+    return NextResponse.json({ error: error.message }, { status: 500 });
+  }
+}
+
+export async function POST(req) {
+  try {
+    const body = await req.json().catch(() => ({}));
+    const requested = Array.isArray(body?.extras) ? body.extras : [];
+    const result = await installHeadroomExtras(requested);
+    return NextResponse.json(result);
+  } catch (error) {
+    const status = error.code === "NOT_INSTALLED" || error.code === "NO_PYTHON" ? 400 : 500;
+    return NextResponse.json({ error: error.message, code: error.code || null }, { status });
+  }
+}