fix(assimp): pass -f<format> so non-extension targets work

The assimp converter invokes `assimp export <in> <out>` and relies on
assimp's extension-based format inference to pick the output format.
That works for targets whose id happens to match a real file extension
(glb, gltf, obj, stl, ply, etc.) but silently fails for every target
whose assimp id is not a conventional filename extension:

  glb2, gltf2, objnomtl, stlb, plyb, fbxa, assbin, assxml, pbrt, assjson

Picking any of those in the UI aborts the job with:

  assimp export: no output format specified and I failed to guess it

assimp_cmd's `-f<h>` flag overrides extension inference, and the values
in `properties.to.object` already correspond 1:1 to assimp's format ids
(verified against `assimp listexport`). Passing `-f${convertTo}` makes
every advertised target work without changing anything else.

Reproduction: upload any .stl, convert to glb2. Before: ENOENT on the
nonexistent output file. After: valid binary glTF 2.0.

src/converters/assimp.ts

@@ -121,7 +121,7 @@ export async function convert(
   execFile: ExecFileFn = execFileOriginal, // to make it mockable
 ): Promise<string> {
   return new Promise((resolve, reject) => {
-    execFile("assimp", ["export", filePath, targetPath], (error, stdout, stderr) => {
+    execFile("assimp", ["export", filePath, targetPath, `-f${convertTo}`], (error, stdout, stderr) => {
       if (error) {
         reject(`error: ${error}`);
       }